My name is Steve Faubus. I'm the head of astrophysics here at Oxford.
And it is my very real pleasure today to welcome Professor Rainier Weiss of M.I.T. as our 64th Halley lecturer.
The lectures go back to 1955, and it is a tradition that we now share with the Atmospheric,
Oceanographic and Planetary Physics Sub Department who host the lecture every third year.
Over the years, we've had a remarkable collection of scientists.
As our Halley lectures, some of them are shown here.
I prepared a very random and totally informal screenshot with no particular selection criterion the
members of the audience may enjoy going through and identifying some of these cast of characters.
I have taken the trouble not to be too parochial and have included oceanographic and atmospheric scientists in the mix.
Now Ray's life to me seems like the stuff of a Hollywood movie,
beginning with a harrowing escape from Nazi Germany in 1932, a very pivotal year and the year of Ray's birth.
After arriving in the US in 1938, Ray spent his childhood in New York City and attended M.I.T. as an undergraduate.
Now he dropped out in his junior year, and in Ray's own words, that's a story for another occasion.
But he did return in 1955 to complete his voice and his Ph.D. in 1962 under Gerald Zacharias,
after an instructor shift at Tufts and a postdoctoral stint with Robert Dickey at Princeton, Ray was then hired back at MIT.
He's a lifer as an assistant professor, where he has been since Ray is a gravitational physicist.
And there are two main threads that go through his professional life, both experimental in nature.
One is the characterisation of the cosmic microwave background, and the other is the detection of what is known as gravitational radiation.
And that is the topic of today's lecture. To set things up.
Consider how one might go about trying to measure the gravitational field of this pair of black holes going around in a binary to make it hard.
That's not a question that didn't come up. That's the earth with the light.
I calls that okay to make it hard to make the binary system 300 million light years away.
Now, you know Isaac Newton's theory of gravity. There's pretty much only one way to do this.
You have to measure the changing tidal force from that binary black hole system.
And the trouble is, that number in centimetres represents the height of the tides that would be created by that black hole system.
So that's one with a like Eddington used to do this and I it's a tradition I'd like to resurrect,
write out all the zeros in a very big number rather than using scientific notation just because it has a much more visceral fat.
So that's very hard to do, especially since that number is second on is the radius of a proton.
Now, in Einstein's theory of gravity, there's another way to measure the gravitational field,
because it turns out that like radio waves, gravitational waves, gravitational forces can be radiated.
And if you do that and you try and set up a detector to measure how strong that gravitational field would be,
then the mirrors in the detector, as we'll hear about from Ray, move that distance.
Now that's considerably higher than the oceans would rise from a tiger disturbance.
But if you find yourself up, you're down by.
1%. The profile breaks for me as a theorist.
I go home. But Ray is going to tell us how through incredible perseverance and ingenuity, he managed to pull this off.
Gravitational radiation was first predicted 100 years ago by Albert Einstein, and it has been since its inception,
an enormously difficult topic, both in terms of theoretical understanding and direct detection.
But it has long been regarded as one of the crown jewels of experimental physics.
And in February 2016, the first unambiguous detection of gravitational radiation was announced to the world.
And 2016 is very much Rey's annus mirabilis in rapid succession there followed with his colleagues Ron Driver and Kip Thorne.
The awarding of five five count them extraordinarily high level international prizes, the Breakthrough Prize in Fundamental Physics,
the Gruber Prize in cosmology, the Shaw Prize in Astronomy, the Cavalli Prize in Astronomy.
And Israel's Harvey Prize. So now it's time for me to stop.
And it is a real pleasure to turn the floor over to Ray,
who will tell us about observations of the merger of binary black holes, the opening of gravitational wave astronomy.
Actually. Hey, that's one [INAUDIBLE] of an introduction, actually.
La la la. Okay. Can you see this thing?
First of all, can you hear me way in the back there? Oh, excellent.
Good. And let me see if I still have a point of view. Oh, well, let me first of all, thank you for inviting me here.
This is a and when I looked at the pictures, I recognised about five people in that picture and they're all fantastically better than I am.
Okay. So this is you know, you're getting you're cutting the bottom of the barrel here for this lecture.
But the I'll try my best. And the other thing I have before I start,
I just want to say I'm saying giving this talk on behalf of a collaboration which is actually Oxford is not part
of it turns out in the in the United Kingdom it turns out and Glasgow is part of it in Glasgow University,
Birmingham is part of it and I believe that is part of it also as is Cardiff,
and this is a thousand people and probably up to about 60 institutions now that are involved with this project.
It's a turned into a monster project and it's so whenever you start thinking about it, don't think of just Kip Thorne and Ron Weaver and me.
That's not the right way to think about it. It's a really huge thing.
A lot of people make contributions to it. So enough of that.
But and so I want to start really with and I was asked actually by Steve Steve advice last night yesterday, well, is this what level?
And he said there should be a popular talk. So I'm going to give a popular talk.
And and if you have questions that are technical, we'll try to get to them as you as you asked.
And you can ask me any time, but then it's not going to be highly technical.
And I'm going to start with this. And of course, we know all about that.
I mean, here you have Isaac and his wonderful equation, which is maybe the basis of most of what we always teach everybody.
Every undergraduate, almost in every high school, grade school, you learn about Newton's gravity and that it works goddamn well.
It got us to the moon. It got us it made a wonderful synthesis in science, which,
of course we make the philosophers of science make much of maybe that the apple which probably didn't fall out of the tree,
but whatever fellow that that Newton saw look to him the same as a thing that was in the heavens,
the moon falling, and that association, which was the first time that man, I think,
ever made that was the thing that led to the idea that things you can measure on the ground are things that also might apply to the heavens.
I think that was the first. I maybe if as a philosopher of science here, you can tell me I'm wrong, but that was a major step for man.
And so this universal theory of gravity is really why it's called now, what's wrong with the theory?
And what's wrong with the theory became evident actually in the middle 1800s already.
But it wasn't then as a big deal. It became a big deal when Einstein came on the scene in 1905 and it was realised
that there was no way in this theory of having things propagate information.
Except an infinite speed, in other words. And we were talking about this.
You and your son were discussing this. And I use it very often in a lecture.
Is that suppose the son disappeared? Don't ask me how that's done.
We would, in principle, in Newton's theory, know it right away.
We would know it because we would go on a tangent in our orbit. But you when you look at Einstein's theory, you don't see it right away.
You see it about eight, 9 minutes later. And that is the big one of the big differences.
There has to be a news function that exists. Once you have special relativity, you can't have infinite information transfer, infinite speed for it.
The other thing is it doesn't work for high, large masses and large velocities.
That was clear already when special theory came along. And so what is this new theory?
And he I'm going to dwell a little bit because this one to most of you who well, this is, first of all, the equations that replaced it.
Not that I'm explaining that equation to you, but I will tell you what the equations says.
And this is that, and I'll give you a little example of that and try to make it a little clearer to you.
What it says is this This is now the replacement of that equation, of the force equation.
It says that the way you measure space and time, which is what this is, is determined by the distribution of matter and energy.
That's the basic idea that's here.
And so you you actually mess with a very important thing, the geometry of space, and you measure with the way time is kept by clocks.
And that then is determined by the distribution of matter.
And let me try to give you a feel for this in this little picture here.
This picture is. How many of you have volume? Most of you are adults.
So there usually there are some kids also. But the thing is that imagine a jungle gym.
I know I come from New York and I know what a jungle gym is. Who does not know what a jungle gym is?
Well, you are you have a few in England. Well, it's a big bar, but your kids play in it and they'll bars that go in the direction,
the Y direction, the Z direction, and they're always separated by a standard amount of distance.
Imagine that you have made erected this in space.
And what this picture is, is a cut through that jungle gym in one and two dimensions just cut across.
That's the first piece you started having laid out space that way.
And now what you do, you have done a little more, which is not in the picture and too hard to draw as you take in.
Here's the jungle gym, crossbars and all of that in one cut the x y plane and then you have clocks everywhere.
Every intersection of these section of these coordinate lines, you have a clock.
They're not in the picture, but the clocks are very important.
And now what you do is you synchronise those clocks everywhere in the jungle gym, and that's not easy to do.
You can't just look at the clocks to do that.
You have to walk around with a clock in your pocket and check the clock next to you to make sure or an awesome theory.
But what you do is you walk around and make them all weave the same time at the same moment in time.
And that's not something you can see with taking a photograph. You have to walk over to it and do it.
And now it is done as you plunk these things like this sun into that system and here's the earth.
And what they what that happened, what that equation will tell you happens is that you distort space.
You make things that were straight lines, no longer straight lines.
And that happens here. And furthermore, you affect the clocks so you don't see them.
But I'll tell you, they make the clocks one slowly around the sun.
They make a do around the earth as well in the region, in here and way over here.
The clocks all keep the same time as they did before as you set them up.
And that structure is the structure that was Einstein's structure for thinking about how he was going to think about gravity.
So what happens now is that those equations that I showed you in the other in
the other view graph are the ones that tell things that are in there like this,
like the earth and the sun, but tell the earth how to move because of the distortions induced in the space and in the time by the sun.
And there's a distortion consequently also of the Earth that it makes its distortions.
And and that's the replacement of Newton's theory. Okay.
Now that theory has gravitational waves in it, and we'll talk about them now specifically.
And so the people before I get to that, though,
I'd like to tell you people who at first saw the gravitational waves and then we'll talk about what they actually are and how you measure them.
This was the house and Taylor that's the famous they they Russell House was a graduate
student and Joe Taylor was at the University University of Massachusetts at the time.
He wasn't yet at Princeton. And they found a pulsar system which actually was told them a fantastic story.
They found a pair of pulsars, which are now many of you or most of, you know,
a pulsar, but a pulsar is a rotating star that's made of all neutral matter.
They can spin quite fast and they have big magnetic fields.
But the important thing here is that they saw pulses from it every time that star went around.
They they were they had a radio dish in a receiver and they saw this pulsing of the pulsar.
And it was at a fairly regular rate, a completely regular read, except it wasn't totally irregular.
What they noticed is that the pulsing was sometimes a little faster, sometimes a little slower,
and they made a model out of that which consisted of that pulsar and another one that was orbiting around it with a period of about 8 hours.
And the pulsing period was about 17 times a second, but it wasn't constant.
Sometimes it was faster, sometimes it was slower. The pulsar they were seeing is this guy right there.
And just from divining that one, from that one observation and many, many others,
but from that one system which they could only see by radio astronomy, they divined the following thing.
And here's a plot which has the very first detection of gravitational radiation in it.
Namely, here is the time it takes to go round. You see these pulses from this pulsar, okay?
Here you see the pulses coming out of the sky and you're plotting the time they arrive at the earth.
And they're coming at this rate 17 times a second. And you plot the the rate as a function of epic.
Here is the 1973 when they started and nine and it goes on and on but in 2000 and they plotted the following what
they did is they watched this the pulses and they got the interval between the pulses got shorter and shorter.
And they divined from this that they that the pulses were going at the same rate.
But they even when you solve for the motion of this thing, the period of the system got shorter and shorter.
And so what happens is, as they saw this curve, which is the rate of the time between,
let's say, going around once each time and around the system, going around, it's orbiting.
And so what happens is that you see these dots that he put in this curve, and it's the line that goes through.
It is the line from the theory that you get from Einstein.
If there is gravitational radiation, it was the real first discovery of gravitational radiation by man.
And it showed that the thought theory that Einstein had was right, that there was indeed a some kind of energy leaving the system.
And and what that energy does and how it manipulates things and how you ultimately detect it was still not yet done.
What would you add that came next? But first, the fact is that the system was losing energy.
That's what it was doing. It was getting the orbits was getting closer and closer.
They were falling toward each other. And there was and it was going slower.
I mean, it was getting the orbit was getting faster, but they were getting closer together.
The energy over it was getting smaller and energy was leaving that system.
And so that then is the first discovery of gravitation. We made a huge impact on all of us and this will make you a little sick.
It also looked up to look at me for a minute and then this is what actually a gravitational wave does.
And this will ponder that a little bit so we can see how a gravitational wave is detected.
But this is indeed that's what in the middle is a point that that's you and this is a whole bunch of little rocks or something you threw out.
And the radiation is this is what is the radiation?
Does the gravitational radiation that comes from a system like that does the following?
It stretches space in one dimension, as you can see, while simultaneously compressing it in the other.
And that keeps flipping back and forth. And you'll see a very interesting pattern in this.
The pattern is that that the further you are away from this point,
that's where you're standing right there, the further you are away from it, the more is the motion.
In other words, for example, from here and there, the motion is large.
Whereas here and there the motion is small that turns out to be.
And that's a compression in one dimension and a and an expansion in the other.
And it keeps flipping back and forth. That's the thing you want to think about.
That's what a gravitational wave does as it impacts on you or as it goes away from you.
And it's a plane wave of gravitational waves. And what it does is that it it is proportional.
The amount of motion is proportional to the separation. That's the big thing.
And it has stretching in one place and the one dimension and expansion in the other.
And that then is the visualisation you should keep in your head about how one might detect it.
And also it's the motions we're going to measure are going to be infinitesimal after a while.
But the thing is, that pattern of stretching one dimension and compression in the other is characteristic of this particular radiation field.
Okay. And so so the measurement challenge in the end comes out to being that these numbers that I mean,
this thing that I show you is grossly exaggerated if you want to measure this.
And the the thing is that once you start putting numbers to paper and we'll talk and we'll talk about that more,
the, the amount of distortion, which is the change in length divided by the length.
That's the string. That's this this is the strain.
That's the gravitational wave strain or the change in length, let's say, of two dots that were far apart and divided by the separation.
That is a constant. That's a strain. So Delta L divided by L is constant over that whole picture at any one moment.
It keeps changing with time, but it's in one dimension. It's a stretch.
In another dimension, it's a compression, that's it. And then it flips.
And so delta l over l the change in length divided by the length is a tiny number for a real source,
and that's where the trouble begins, in other words, for any kind of source.
And here is Kip Thorne, who first really put some of this together.
And he was one of the first people to think about what might be the strains in from astrophysical sources
and what might be the amplitudes that you might be having to look for in a wonderful bunch of articles.
Back in the in the seventies, Kip was well, he looks he doesn't look that way anymore, but he looked a little like a hippy.
None of us look the same 30 years later. But the thing is that he already knew.
And it is when you look at that book, that's Mr. Thorne and Wheeler,
you'll find that the numbers for strain that any kind of astrophysical system that we could detect have you'll be in the regime of ten to the -21.
Now those of you who are not familiar with that expression, let me just over here show you what I mean by that.
It's much easier for me not to do what what I thought you did, because I can't do it.
I have to keep writing these numbers with enormous numbers of the.
Zero five adjustment point. So it's certainly important to know this.
But the number, for example, is ten to some number you.
So here's the way to get familiar with it. A thousand can be written as ten times, ten times ten, and that's a thousand.
But it's also going to be written as ten to the three. In other words, three times power of ten or something small.
When you put it in the denominator, ten times, ten times ten, the denominator, and that's ten minus three.
So these are the numbers that are associated with that. 1000 is this big guy and a thousand is this little guy.
And so I have to use that. I don't I can't I can't go on because we have too many tiny numbers that we're going to use.
So as anybody lost with this, tell me. I'll try to do better.
Okay. So right. So the so the thing is that the kind of thing to expect from gravitational waves.
And that comes from calculating, you know,
formally what they might have do might might do is sort of in four kilometres you might get a change in length.
That's from this of a system of about four times 10 to -18 metres.
So that's just exactly what if you know what you were just told.
In other words, it's a big, big number with a lot of zeros in a decimal point.
And that means that you're you're going to have to sort of make a technology that can do this.
And here sort of is one way to highlight it all.
The the the the length change is that you have to measure to get into business are sort of ten the -12
of the wavelength of light that's in order to measure these gravitational waves you have to do that.
Or the if you or and if you want to make sure that you are not exciting things,
other ways in other words here is that you have the ability to detect them and you don't shake things artificially for other reasons.
You have to be able to make motion measurements that are part of ten miles, 12 of the vibration of the earth at the Earth's surface.
In other words, when trying to say it's a very small motion that you measure change in length built out.
And also you have to protect yourself against all the motions that might interfere with it.
And that hasn't actually was already known by Joe Weber and others early on.
And it makes the thing a difficult a difficult science.
And so I want to give you a sort of a feeling for how you go at this.
Okay. And this is if I see that you get bored and you look I'm feeling like children or I'll stop.
Okay. But the thing is this, that, you know, here is sort of a hierarchy.
And then, you know, if you talk about, you know, a metre.
Well, all right. Here's sort of the kind of things you can still do easily.
Stop right about right about here.
In fact, you can yeah, you can still do well about there.
In other words, you divide by 10,000, you get to the size of your hair.
If you have four, if you have one metre, then you divide by another one and you get to the wavelength of light.
That's still way far from where you have to go. We're going down to 1018 metres, then you divide by another 10,000,
you get down to the atomic diameter and you can just about do atomic diameter by simply measuring how a drop.
This is an experiment you can do yourself. You can take a medicine.
Drop it. This is sort of an interesting thing to do with the kids.
You drop it and you drop it into a big puddle of water and you look how it spreads out in the water.
Take over. It takes some, well, slightly fatty acid, fatty substance,
some well orange juice will work as long as you can see it and you can see it spread out over the water.
And then you get a thing which has a certain area and then you can get from that the height because you know,
the volume of the drop that you started with and so the volume of the drop,
you take that same volume and distribute it over an area that you dropped into a puddle of water.
And you if you calculate if you measure the area of that, which is easy, just like a ruler measure to do, you'll find a height of it.
And that height of it is going to be of the order of what, ten months, eight, ten months,
etc. will be of the order of ten minus ten metres and like eight centimetres.
But from then on in you're stuck. You're going to have to believe me. I mean, and so you're not there yet.
You have to go by another 100,000. You get down to a neutral diameter.
And finally, a still another factor of a thousand to get down to what is needed to make a measurement of these gravitational waves 10 to -8 metres.
So I don't want to I only did that so you can get a feeling for what's involved here.
It's possible to do those measurements, but you have to learn how to to do it.
And and so here is the key.
I want to show you a little bit in an animation what a gravitational wave does here.
This gravitational wave is coming down on that system, which I'll turn on in a minute.
This will get active in a second and let there be. It is a laser here.
This is just all made up and there's a beam splitter here.
This is the way, in fact, the instruments work that do this. The the laser will hit the beams, the light.
To reflect from that mirror. Come back and reflect from that mirror.
Come back. This is a part partially reflecting mirror. And here's a photodetector.
And you'll see that if the time that light has taken is exactly the same,
having hit the speed of going back and forth and hitting the piece between having gone back and forth,
there will be no light going to this photodetector and that's you'll see the basis of the infection will get here.
Sort of let me get the animation running. So here is that wave is the wavelength of the light.
Wherever it is, red is there is intensity of light.
And this is one arm and here's then the other arm.
And now if these paths are exactly equal, no light goes to the photodetector, which is that place right there.
The two beams cancel each other. And now you start moving these guys, which is what the gravitational wave will do.
They'll move. This will move in. That will move out. And then keep doing this push, pull motion.
And then light does show up at the photodetector. And that, by the way, is the basis of this, the gravitational wave coming down on the system.
That is the basis for the entire observation. It's no more than that.
Okay. And so. All right.
Now let me give you this is sort of people who got involved in this early on.
This is a sort of rogues gallery of the people who began to think about this technique for measuring a gravitational wave, which does exactly what is.
Now, I showed you in that picture. It stretches one arm as it comes down and license to the other.
It does this push pull motion, which that, you know, particularly sensitive to with that device, which is called the Michelson Interferometer.
And so here, for example, here is this is a group at MIT.
This is back in the you know, by the way, the guy who really thought of this first as I don't know how many of you know him as a piranha,
is that a name that means anything to anybody in the room? Some of you know him.
He was a relativist who worked in England and was the first to really recognise that it was possible to make a measurement of gravitational waves,
which we'll get to in a minute. Usually not using lasers necessarily.
They didn't exist really at the time.
This was all in the 1950s, late fifties, but that you could actually try to make a measurement of a gravitational wave,
which does stretch space in one direction and shrinks it in the other.
That's really the motion, just the motion that we were just seeing here. And he wrote a wonderful paper in 1957.
And all these ideas for how you do this actually are derived from his theoretical work.
And so here, for example, is a instrument that I want to strive anymore to say this was the first one that did this.
Here's a laser that the lasers had to be important.
And then you have one arm which is light hitting a beam splitter, just like what you saw in that movie.
And the light in this case bounces back and forth many, many times. That's what these curved mirror does and comes out again.
Same thing happens here. The light goes through, this mirror bounces back and forth.
There's a hole and then it combines and you get just what you saw in that picture,
either light at a photodetector or not, and you set it up so there's no light at the photodetector.
The gravitational wave comes down, and this already was the germ of the idea back in about 1972.
And then the people who really made it happen, this was sort of an idea.
We built a little one at MIT. But way after I mean, this was with students and and now people who grew up with this.
But the people who really did it and made it go were people in the Max Planck Institute about in the seventies or late seventies.
And here's here's Shiller. Here's a guy named this is a guy who ran the group in in Germany.
And these people all contributed. They were all engineers and they were the group that did this was making.
I'll tell you what they were doing at the time. Huntsville ran a group at the Max Planck Institute that was making computer memories,
and as a hobby, they did this and then they wanted to do something fun.
And of course, the Max Planck Institute allowed people to do that as you were.
If you were Max Planck director, you could pretty much do what you wanted.
And there was money for it was a wonderful system and all of these people invented different ideas that solved some of the problems of this technique.
I won't tell you what they all were. I think one of the more interesting ones is that of how shooting first of all, I can read it from here.
So. Whether it's her name, she's not yet invented.
The idea that you don't make the arms exactly equal so that you don't have only a dark fringe level.
Every one of these contributed a clever idea to make this happen simultaneously with that in England,
it was a group that was working under Rand River and doing the same thing, but with slightly different technology.
And each of these people were working with Ron. They were making an interferometry detector like this as well after they had built a Weber bar,
which was a bar that I have to left that piece of the history out.
Joe Weber and I want to say much more about it,
thought he had discovered gravitational waves in about the middle sixties and this was a different idea than his.
This was an idea using in a from a tree to do the measurements of a change in length in two perpendicular arms.
What Weber did is use big bars and he want to see their change in length and he read
them out by using PG keys and devices that measured the length very carefully anyway.
But this group added all sorts of wonderful ideas. Both of these groups actually added ideas to the thing and made it happen.
And so here then is actually the device that later and this much device like the one that made the detection illegal.
And let me walk you quickly through that. So here is now the laser.
And what you've already seen is the beam splitter and that mirror.
That was the thing you saw in the animation and that mirror. And then these were the mirrors added to make it.
So you bounce the light back and forth many times to gain sensitivity as these are FRB probe cavities, as they call them.
There's another one right here. And then there were some very clever ideas that came up from different groups in this thing.
For example, the German group and the Geneva group came up with the idea of putting another mirror right in here.
That's a mirror between the laser and and the interferometer.
This is this in a primary thing. And that is the mirror sounds like it blocks everything.
And it's a very smart idea. What it does is it takes it.
So if you take light and you have no light going to the photodetector right here, you've been made a no fringe, no light at all.
Just like that movie showed you. It was no light when these paths in here were equal and then the light goes off, nothing goes here.
All the light goes back to the laser. That's where it goes. And then what happens is you put a mirror here, which is partially reflected.
You can do something really quite elegant. You can make it so that the light that goes back to the laser is reflected in part back to
the instrument and also is added in such a phase that there is no return light to the laser.
In other words, the reflection that maybe from this light at this mirror is cancelled by the light
that would be partially transmitted by the light that comes out of the uniformity.
That's those devices. And then from it, so you get no light back to the laser.
So you can take, for example, a 25 watt laser here and wind up with something like kilowatts of light in
this little cavity in here and megawatts of light in this cavity right there.
And the gravitational wave is coming down and it's stretching one of these and contracting one of those doing that picture that I showed you.
And so that was the first big invention that was made to make these things very sensitive.
And then the much more subtle invention,
which is to put a mirror between the detector and this is where the detector is and the interferometer itself.
And that's called a signal recycling mirror.
And I only will tell you what that does, and you can ask me that in questions later, because it's kind of complicated.
This one's pretty easy to understand.
This is builds up the power in here enormously, but this one can be viewed to tailor the spectral response of the instrument.
So when a gravitational wave will do is it'll stretch one of these guys and shrink that guy.
Some will tell you they keep moving back and forth, make the caddy's longer or shorter,
and that can be detected by light appearing at the photodetector when the cavity rings are not the same anymore.
But they use you, gives you a clever about it.
You can even feed some of like black light back into the system and make the system have a different spectral response.
You can change the way it responds to a gravitational wave, and I won't go any further than that.
And then to say that in this picture at this point, so these were ideas that were generated by the people in those pictures that I showed you.
And so that is, in fact, the instrument that that that that made in fact,
this the instrument that made the detection that you heard about that you just heard about.
And so now there are other things that you have to do, and I won't go to all of this, but once you have a device like that,
you have to make sure that the fact that the ground is moving doesn't bother you because you're measuring,
as I showed you earlier, tiny motions, ten to the -18 metres and not ten minus eight metres.
You can't you can't touch that to the ground. The ground is shaking much too much for that.
The ground is shaking easily by a micron everywhere.
So you have to do something to eliminate that. And that's done with very complicated suspension systems.
Here's one of them. This is these are a way of that. You have these springs up here hung from the ground.
And these are all individual pendulums. This system is a pendulum to that.
That's a pendulum. That's a catch. The series with that one. That is a.
Another pendulum attached with this one.
And finally, this mirror is one of those end mirrors of the system which has allowed you to measure the distance.
So that sort of thing was invented by people in Britain.
It was a very, very good idea. And then there was a thing done in the United States for this kind of noise,
which is a little more difficult to explain, but it's very much like the following.
It's like how many you know, about the the earphones that you can have that don't let you hear something.
When you were going in an aeroplane, for example, you know, the noise cancelling earphones that is.
Anybody work with those ever. Yeah. If you have them, you know they work by having a microphone and that microphone puts in an end the sound but
on the opposite face to what the actual sound is that's coming from the sound field itself.
So that's the same thing that we did with suspensions. We made them so that you could and this is sort of one of the ideas that was that you float,
you float a suspension, and then you drive it in such a way that it's a mirror that might be mounted on it.
It does not move with respect to the ground because it is cancelled the motion of the ground with seismometers.
So the mirrors never feel the motion of the ground because the motion of them gets
cancelled by motion of seismometers that are on the same supports as the mirror.
So that makes it so that you can live in a city. You can live in a place.
Well, it's not it's not a wonderful way, but it's a way to get rid of some of the ground noise.
And here is a much more complicated picture of the same thing, just so you can see what it looks like.
This is actually essential. Here is that suspension that I showed you here, for example, is the laser light in the interferometer.
And this is hanging from this system, which is this is the first one I showed you with all the difference,
this pendulum and this thing here is a there's this kind of system where you reject the seismometers
that measure the motion and reject the motion with seismometers that would have driven the system.
So it's a way of sort of compensating for the fact that the ground is moving and all of that was necessary to make these measurements.
And here I just want to dwell now, this is a little technical.
I want to dwell a little bit for people so they understand where we're stuck and what this picture shows.
And I can't tell you everything about the picture, but what this picture shows is the performance.
This is frequency this way and this is strain, the gravitational wave strain.
This is a that h that amount of change of length divided by the length, which is the gravitational wave signal.
And this is four different interferometers. And this is the noise, for example,
that the first interferometer done with this red one is the first in a front that was done by the IT back in the early nine,
the early part of this of this century.
So 2005 to 2010 about this is the proof that was a performance of a of a of a system in and in Italy called light.
I called the Virgo. And they were running with us.
And here is the one that was legal, which is the American version. And that had two sites, one in Louisiana and another one in Washington state.
And this is a performance. You can see it's not as good. This is at low frequencies.
This is about a this is ten hertz. This is 100 hertz.
That's a kilohertz right there. And then when we rebuilt the apparatus and made it better, we saw nothing, by the way.
Absolutely nothing. And no gravitational waves doing anything with that first instrument.
And then we built it better. And the first curves that were this is when we still are living with is this one with this green one.
And you can see that is a lot better than any of the other things we had earlier, but it's still so difficult to read.
The exponents on the left is the very top. Okay.
I'm having trouble myself reading it, but this is ten to the minus.
24, 20 miles, 23, 20 miles, 22.
Okay. And what are the units are? These are displacement.
These are metre, the delta L overall. So it's a dimensionless quantity per perf frequency band, the square root of frequency.
Okay. So you're talking about frequency spectrum here. I didn't want to get into that.
I'm just glad you asked me that. And so what what comes of it is that here, for example, these are the things that look like.
No, they like beautiful theoretical curves are indeed theoretical curves.
And this is the thing that, you know, these are real data.
And this is what, for example, a instrument which is in Italy, which is the Italian version of a thing you're seeing here.
And the ones with all the real data are the ones that are from Legault,
which is the one that's the American version of this is these are the predictions for what they should be doing now with their instrument.
And here's the prediction for what we should be doing now with our instrument.
And you'll notice that we are not doing as well as we want to.
The green line is where we are, and this turquoise line is where we ought to be for that setting.
And this is the very best we can do with the instrument that we built now, both in in Louisiana and in Washington State.
So we had this a gap, and that gap is not understood, and that's worrying us a lot.
You might as well know that.
In other words, the instrument does not work at theoretical performance that we think it ought to, but nevertheless it made a detection.
And by the way, what's down below here are and I won't go into any more of that is things for
the future people thinking about how do you make the configuration better.
And there's another factor of almost sort of between 20 or so between where we are.
If we get to this and we are finally we think we cannot do any better and that's the thing for the future.
So these are then and so what are these things? So, you know, let me give you a little movie of it.
These are the two sites. This one, these are the American sites.
One in Hanford, Washington. That's right.
Where the atomic well, the atomic bomb material was the made I mean, at the Hanford site.
And that's a big, big national site.
And the other one is in Louisiana and it where it rains a lot and it's a problem and there are others.
We'll get to those in a minute. And those two detectors are the ones that have done all this work.
And here's sort of a quick movie so you can see of this. This is the one at living in Louisiana looking down on it.
And the there is a central building.
This is the one in Washington state. And this is these are culverts that cover the things for wind and rain.
Here's this one in Louisiana, same thing. And here's what the beam tubes look like.
They're about 1.2 metres in diameter.
And here is somebody working on a laser cable, just the kind of cables you probably have in your laser labs also.
And here's the control room for that in Louisiana and people being taught how to operate the instrument, which is not trivial.
And so. Okay. So what were the criteria for making a detection of gravitational waves?
And that takes a little explaining. You have these two you have these two detectors, one in Louisiana, one in in Washington State.
And the very first thing is that if you want to say you've detected a gravitational wave,
you've got to say that you've seen the same thing at both places.
That's absolutely critical.
In other words, you have to see to say the same thing within the time difference that might be appropriate for a gravitational wave.
And that difference is dictated by the fact that we believe and I think now we have proved that gravitational waves travel with the velocity of light.
So that says and since you don't know where they're coming from, you have then with these two sides,
you have to leave yourself a window that's opened by something like, you know, the order of some numbers of tens of tens of milliseconds.
Okay. The other thing you have to. So that's very important. You have to see the same thing in both places.
If you don't, it's it's off. The other thing is that you also want to make sure that you are not disturbed.
And these tiny motions that we're talking about here by something that is physically happening to the environment.
And that's a whole you have a whole set of of devices that measure, you know,
the ground motion, the motion of the chambers, the tilt of the ground, the noise.
And then we'll go into magnetic field fluctuations, line build off of wind speeds.
All this stuff is needed. And you want to make sure that anything like that exceeds a limit.
We don't accept the result,
but that result and that's already one way of cutting back on events that that are going to cause you false coincidences between the two sides.
What you're looking for is something that will appear at the two sites within 10 milliseconds.
Okay. That's the detection. And finally, what you're looking for is that there can't be any.
There are a lot of other things in the detectors which should be going on.
And for example, if you look at the alignment signals, you look at other you look at the frequency variation of the laser,
you look at amplitude, vary all the different things in the parameter, which you can measure locally.
You make sure that those are under control.
So if it satisfies all of those conditions, namely the ones that are there, you begin to think that you might have detected something.
And that's sort of the the the the routine by which we operate.
And what we're seeing is the very first thing we saw back in now was in September of
September 14th of 2015 was that long ago we saw this and we saw and this is now.
This is time. And this is the amplitude of the strain that's delta l over l the change in length divided by the length of that system,
as well as configured as you saw. And this is all noise.
And then suddenly stars, things begin to happen and they're coherent. And there it is.
This was what was seen in Livingston. That's the Louisiana one. And this is if you get an idea, this is ten to the -21.
It's a strain of 10 to -1 is and that's one times ten, which you want.
So here's the thing seen in Hanford, it's not exactly the same time as you'll see in a minute.
It has to be displaced. But there, you know, this is noise.
But then you begin to see something that although the noise doesn't get you to make it perfect,
you can put them on top of each other and they and slide them.
And so this is what was done. And they don't look completely out of hand.
They look like the same thing within the noise that is in these instruments.
Okay. And that was our first detection. And the thing that caused us a lot of worry because we were not.
And here we are going to show this is another way of representing that first detection.
And this is about time versus frequency. And for the two sites, this is Livingston and that's Hanford.
And and and this runs from about 30 hertz, sort of just from the piano bar on the bottom.
The piano is about there. So the 21st and it goes up to you can see there is the signal is a frequency
as a function of frequency hears the signal time as a function of frequency,
it goes up to about oh middle C to 56 that's at, at Livingston and it's not very different at at Hanford.
Those are the two sites and you make them superposed and they were needed about 7 milliseconds apart.
And there they are. And here's what it sounds like. It sounds like a dull thud.
I don't know if you can hear it. Uh, probably not. I was this as loud as they can make it.
So we did a little. Conniving with this. Okay.
Maybe you heard a little of that, but those were coincident pulses that you heard delayed by about 7 milliseconds.
That's because that's it turns out the signal got to the way Louisiana first went through the earth and a shell showed up in Washington state.
Okay. And so that was actually very important to us.
And so that was the first detection of of a wave.
And let me show you what's going on in a model of what this these these models took many years to develop.
And I don't know if any of you were familiar with the struggle that people had in
trying to solve a general relativity problem with real data and real numbers.
It's not easy. It was one probably the most difficult thing in the field, almost as difficult as the experiment.
I mean, it took years to get of a solution to the Einstein equations that you could then
actually carry out for a system like what you're about to see in this movie.
So that was a big triumph on its own. And here's what that looks like and what you'll see.
You'll see the signal on the bottom and then you'll see the actual the same kind of cut that I showed
you when I showed you the picture of the the the earth and and the two dimensional cut of the space.
That's what you can see under those masses. So I'll I'll talk you through it a little bit.
Okay. So here are the two objects. And this is supposed to be these objects that you just saw.
Oh, we as we saw the real data from and this is time up here and you'll see that these arrows in here are the stresses in space.
And you can see the the the holes are effectively the distortion of space by these two objects, which are two black holes, it turns out.
And we'll get, we'll see. They get deeper, deeper.
And the distortions of space, which are the stresses really also you get longer, you'll find, you'll see.
And this is then I think they'll slow the thing down in a minute.
And you'll see actually that this is space and time is what is is being represented by a real time.
And you'll see the structure forming between these two black holes.
And this is you'll see looking end of time as we almost stop completely in there.
And this is now they stopped it just so you could see it better. And now those two black holes formed, one single black hole.
That's what we thought we saw. And that comes that is in that particular signature is in fact, what the signal we did see was.
In other words, it mapped as best as we know, what all the different attributes of the signals that you saw.
And you can't do it perfectly because the signal noise is isn't isn't huge.
Signalling wise was about 20 to 1 so it was plenty good enough to make the to make the detection.
And so that's what was seen in two black holes.
And here is sort of a little summary of all the others, and I'll try to make it clear to you what it was that we're seeing in the in the experiment.
Here's the one we just saw the the animation for. And that's the signal that we're seeing here is what it is, is this time.
And this is the strain. Each one of these little panels is his own strain.
This is ten and -2110, the -21 plus one times nicely, minus one time standing by 21.
And that's the thing we just saw. We saw the thing. This is the space between.
Shrink. And then what you see is this thing going, getting faster and faster, and then all of a sudden it quits.
And that's the quiescent state. That's the that's the first signal we saw.
And that's a big one. And I'll get to talking about that in a minute. But then the next signal we saw wasn't.
So this was done by actually looking at the signal. This was done by a different technique was smaller,
and it was done by actually and this is the way we thought we would have to do all of these things.
The first signal we saw that we saw by I have really no difficulty in seeing it,
but the next thing was you make a you do a cross correlation between different
templates that you solve this problem through the general theory of relativity.
You solve many, many,
many versions of it and then take your data and see what is the best cross correlation between that signal you see and the calculation you made,
a numerical calculation you made for general relativity,
and that best situation is the one that's sure that gets you these parameters, which we'll talk about in a minute.
So this one is marginal. This is a five sigma signal, five times the word, you know, the noise that we saw is five times.
It has a uncertainty that is associated with a signal that's five times bigger than the noise.
One way of thinking about it. This is also something like a fourth sigma signal, but we're not sure of it because of some problems.
This one we're sure of. This is a different set of masses, as we'll see in a minute.
And this is the one we just detected, not more than about what we detected it back.
You can see the dates. This is the January 4th of this of this year.
And that's how these things work. And so this is just recent. I mean, it takes a while to get confidence in these.
And here are some of the results of these signals.
For example, this guy was had a mass of 36 suns and another mass of 29 sunspots with a two, two black holes.
And they lost a mass of three songs.
In the process of doing this, the binding is just enormous.
In fact, that's more energy that that was stronger than anything in the in the universe for a brief moment,
that amount of gravitational wave energy that left that system.
And so these are big black holes.
We don't know much about those. And the black holes we see in our own galaxy are sort of few.
You know, this is a by the way, this system is a billion light years away.
Okay. And the black holes in our galaxy, there's one central black hole that's about ten to the five masses.
But there are black holes in X-ray star X-ray binary stars, and they run around five, four, three solar masses.
These are big guys. And that was one of the big mysteries.
None none of us expected that. Uh, the most recent one you can see, the most recent one is another fairly big one.
These two you had to do by signal analysis carefully.
These were obvious. These two were obvious when you just looked at the output of the of the instrument.
The other ones you had to do careful signal analysis to to see.
And so here's an interesting thing that I sort of this is sort of tells you a little bit of the awesomeness of this thing.
If you put that sort. Munis, 36 masks, 29 mass source.
And before it condenses into a thing, that is the sum of those two minus three.
Okay, then you put it at one this and put it the distance of the sun, the strain that ten and ten,
the strain which is the length change divided by the length delta l divided by l would only be at the earth ten to the minus six.
You would hardly notice it. In other words, it's an enormous thing.
Huge amounts of energy are disappearing, but you would hardly notice it.
It's a million. I mean, the fact that you expand and contract by one millionth of the year of your one millionth from here to there or your height,
you would never notice. It is zilch.
And nevertheless, what's passing through you is something like ten to the 25 watts, four metres squared as the energy that's coming through.
It's just unbelievable. It just tells you how stiff space really is.
And that's sort of was the miracle of that setting. And that's why these these are enormously energetic things, but they hardly do anything.
That's the real problem. And this one here is it's not.
So this is these these two guys are the biggest ones we've seen.
And they've sort of opened a whole new field of worrying about what is out there.
And in fact, now we can begin to think and I'll talk about that more at the end of trying to do astrophysics with these things.
They are remarkable signals that you can use. They're not just, you know, this one shot thing for looking at one piece of general relativity.
And here's sort of a cute thing that happened after we published the first result.
I was in a subway in New York and I saw this thing here.
I just read it from here. It says that these took people saw the picture in the subway and scientists found gravitational waves in outer space.
If it only were that easy to find an apartment in New York City with a walk in closet.
Okay. And of course. And then The New Yorker.
This is a New Yorker cartoon which says, was that you I heard just now?
What was that? Two black holes colliding. That's what this guy's saying about that.
So anyway, it hit the public imagination in a wonderful way.
And I can only tell you why I sort of I've been involved with other discoveries in and many of them never make this kind of a hit in the public.
And the reason is simple. It involves Albert Einstein, which is he's still a hero.
It's very clear. And it was black holes, which are of the comic books were full of black holes.
I mean, every kid knows that about a black hole and how awesome it is.
And they don't know what it really is, but they know that it's terrible and big and each thing and it sure does that even further too.
So anyway, so anyway, here are some people who are very important in this and I'll just quickly tell you who they are, many of them.
You know, Barry Barish was the guy who finally, after many attempts, several attempts, put this thing together as a real project.
And Stan with him was his deputy in effect.
He was the he was the and Gary Sanders was the engineer that worked with Barry and Stan to pull the whole thing together.
And these are people who were this was these were people who this was these these
were people who helped organise all the engineering effort to make it happen.
And our lives are really and and then this coin and now it gets as time goes on, people get younger.
And these are the other people who are important. This group of people are people who ran the Ligo's scientific collaboration during this time.
Peter Saul was the second spokesperson of that collaboration,
which has about now has about 900 members in it, maybe between 900 and a thousand members.
And David Wright was a second one. And then Garvey Gonzalez was the one you probably saw on television.
He was actually just the spokes woman for the whole collaboration in the early days.
And she now has been replaced by David Shoemaker, who you'll see there he is up there.
This and this is the people who were head these were heads of the project tried besides Barry Jay Marx,
who was a high energy physicist, and David Wright, who is a optical physicist.
He's a current he's the current director of the project.
And these people are the people who actually are he and much younger he responsible for making the system work.
There's Denis again. He's the chief engineer.
Peter Fritsch, who wrote that very elegant paper with a he ran a committee besides being the chief scientist of understanding all the technology,
he wrote that paper, that the discovery paper, which was an elegant paper.
And then these two people and David Shoemaker, who is the he's a he was sort of the chief scientist for the development of the detector.
And then are Frolov and Daniel Sigg, who were the scientists at the sites who kept the thing going.
So these are the people responsible. I mean, the officials who are responsible for making like putting Lego together and making this happen.
And I won't go into anything more to say that we have things we can't do.
Well, I'll just tell you what they are. One, things we can't do well with this is to find out where the sources are on the sky.
Because we always see we see this shrinking and expanding of space in the two detectors.
And all we have is a time difference between the two detectors and where the sources are.
Then, as well as you can make the time difference and you don't get a directivity out of it.
And so this is easier. This is a map, for example, of the other three sources and not all the sources on this.
This is a map, a celestial map as the north celestial pole, south symmetrical.
And this is the uncertainty we have, for example, for the difference.
The most important one is the blue guy. This guy. This is a symbol of 500 square degrees of sky.
If you want to tell an ordinary astronomer where the [INAUDIBLE] to look, you give him a thing is go look there and they laugh at you.
It's just too much. They can't do it. And that's true for these other things, which are not even as well defined.
So we don't yet know how to couple ourselves to the astronomer astronomical sky, that more conventional sky.
We need to have better position information and the way to do that is to have more detectors in the world.
And here is sort of what is being planned.
This is now everybody who's we know of was working on this year the two detectors in the United States that made that these measurements.
There is this detector in Europe which actually is running Virgo, which is a French Italian collaboration.
And that detector was unfortunately not yet running when we made these initial discoveries.
They were part of running with us in the early years when we were getting that zero.
They helped us get the zero. But unfortunately and so we're hoping they can get on the air and still run with us this year.
And maybe we'll have a few more candidates and they will be drawn into it, too.
And that's the other system. And it turns out there is a a interferometer in another one of these detectors up in in Hanover, in Germany.
This is mostly a research attack detector.
These people with the people back that I showed you earlier who were involved in a lot of the ideas for this detector.
And then there are projects going on right now. One in Japan called the cargo detector that's in the Oka mine,
same place where the neutrinos were detected and the neutrino oscillations were detected.
And there is a project called Lego India, which is sort of embryonic, but it has an American built detector,
which was I won't go into the history of that and you can ask me about it, but it's a detector they have taken.
We've given them a detector that was built by the Americans,
but they are paying for the infrastructure to put the thing together so that there would be now,
all in all, something like four and maybe five year five detectors distributed to the world.
And that will help enormously in pinning down in the sky where things are.
And here's sort of what I mean by that. Here is a localisation pattern of having only the Hanford detector.
That's the one at the one in Washington State, the one in Louisiana and the one in Italy, if it were running.
That's the Virgo detector. And this is the uncertainty for a particular class of sources.
And you can see they're quite large, a couple of places in the sky where the uncertainty circles are small,
but most of the places in the sky, you have a you couldn't tell an astronomer really very well where to go.
Look for the electromagnetic analogue of this and.
So here is sort of what happens if you have another detector. This is what we're planning is the one in the one in at Hanford, the one in India.
If the one and then this is the Virgo, one of the two.
That is the this is the Hanford. This is the one in Louisiana.
That's the new one that would be in India. And that's the one that already exists in Italy.
And then you can see that these are now sort of in most places about the size of the moon.
In other words, even that's pretty fearful for most astronomers to tell them, look, look,
in a region around where the size of the moon and look for something exciting and interesting.
But that is a [INAUDIBLE] of a lot better than what we have now. Okay.
And there'll be a lot more interested in helping out if we get to that point.
Okay. So I'm almost done. The the thing is, I just want to talk about other classes of sources and what kind of things people have thought about.
The things I've been showing you where the only sources we've detected. They're all black holes.
Every one of those was black hole, black hole pairs and which is sort of exciting that they exist.
We in the early days, we only thought of neutron star and neutron stars.
There was a star which we will eventually find. They're weaker than black holes because there's a lot less mass associated with neutron stars.
These black holes are a huge thing. And so it turns out these are other sources that very well we will expect to see as life goes on.
So here are the ones we're talking about. Black hole. Black hole, Piers. That was the ones we had detected.
We hope detect neutron neutron star pairs and even black hole neutron star pairs and then supernova.
And let's say this was sort of the beginning sources for the field.
Back when the field was invented, people thought they would look for supernova.
But trouble is, supernova don't happen that often. They happen sort of once every 30 to 100 well, between 30 to 50 years in our own galaxy.
And hopefully we will see why that was really most impressive because gravitational radiation comes out from the inside of the star,
does nothing stops. And so you will be able to see actually what's going on inside the supernova explosion.
So that'd be quite spectacular. But the trouble is, you know, if you want to see many of these,
you have to make the detector sensitive enough to see all sorts of other ones further away from us.
Well, black hole normal it's proposed went to everybody so that we will see or maybe already have begun to see.
And then searches that are triggered by electromagnetic things.
Here is a search which has been going on for many, many years, as long as Ligo's been in existence.
And that's to look for sources that are continuous wave sources.
And for example, you know, all of the for example, that pulsar I showed you, that was the thing that Hudson Taylor found.
We've been looking for it's gravitational radiation from the beginning.
In other words, it oscillates at about 64. It's 58.9 hertz.
And you're looking for a signal at at half that a double that frequency.
Okay. And that and then people have looked for that, carefully scanned it.
They don't find anything yet. But every year, because you can keep running and running and running, that's a source that's continuous.
You can keep looking for it. And eventually I think it'll be found. And they're not pulsars.
You don't have to know the frequency. You can do that without. You can do that over a whole frequency range and look for things that are periodic.
So that's a very interesting source, but nothing has yet been found.
And then a stochastic background, the background of just gravitational wave noise and there you might very well find and that's
something which you may very well find that you see something from the primaeval universe,
but not with these detectors. But it's very much hope for you probably know about the Bicep2 experiment.
I'll tell you about that in a minute. And that the thing is that there may be unresolved sources that you will see.
So anyways,
these are why why I show you this is because each of these items has a group in the collaboration that's looking for that particular class of sources.
Okay. And so it's become a very significant field.
And so here is sort of the last slide I want to show you is the sort of what the field consists of.
This is the field of gravitational wave astronomy.
And so what this is a slide of is is a following here you have frequency starting at about ten kilohertz,
down to ten to the -16 hertz at sort of one over the low over here is sort of one over the age of the universe.
And here then is the value of H, which is that strain, the gravitational wave strain that runs from ten miles, 25 up to about 10 to -5.
And here's like, oh, this is the thing we've just been talking about.
It has this band sort of from ten to 1 to 10 to the four, and it sees things that are compact binaries.
We've been talking about those a lot.
Pulsars, everything I've told you is in that is the written down here is a project that is about as old as Ligo's.
This is a project that NASA has been doing and the Europeans have been doing just about since 1974.
It's to put three satellites up in space. It's called Lisa, the laser interferometer space antenna.
And it the Americans dropped out of it because of the complexities of paying for it.
And for the Hubble telescope. For the. For the new for the new telescope as well, the one that is replacing Hubble.
And but now they're thinking of getting back in again. And this has the problem again, laser interferometer system.
Only the three satellites using laser light between them and in effect is interferometric.
We measure the separations and watch the breathing of that triangle.
That's the way it works. And you have all sorts of you look for big, massive black holes, collisions of small things with black holes.
And that's a very interesting place. If you're interested in other theories of gravity besides Einstein theory,
that's one of the most likely places you will find anomalies by looking at a small object falling into a big black hole.
That's a wonderful place to test the theory, and that's may come may come out of this thing.
And they will see a background which is sort of curious of the our galaxy is full of white dwarf stars that just loaded with it.
And they when they go around each other, they have periods. So you can see the periods of sort of between minutes to hours.
That's sort of the periods we're talking about for this. And these stars have periods in that band and they will clearly be seen.
And when this was initially proposed, people worried endlessly, oh, my God,
we will be completely swamped by the noise of the interference from all those sources.
Well, you know, they should loose along the what they're going to see those.
And then here is two projects which are going at the moment, and there are much longer wavelengths, low smaller frequencies.
One is called pulsar timing, which I think you probably know a good bit about here, and that is that you can use this a very clever idea.
What you can do is you can look for gravitational waves in the band from some number of years to probably halves of a year.
That's the and by doing the following, you look at pulsars in our own galaxy very carefully time them and they're all over millisecond pulsars.
Pulsars that sort of oscillate it 600 times a second, something like that.
And if you have enough of the distribution of them, you will find an interesting thing.
You will find, for example, if a gravitational wave comes through our galaxy with periods that are long enough, so you can watch this,
you can see all the pulsars that are in the northern part of the sky and the ones down in the southern part of the sky will get a little bit faster.
Okay. And the ones in the eastern part of the sky and the ones in the western part of the sky, they will get a little bit slower.
You look for that quadrupole or pattern in the and in the timing of the pulsars.
And that is a way of detecting the gravitational waves now coming through the galaxy, our galaxy.
It's a very elegant experiment. Nothing yet has been found, but they are working very hard at it.
And they I think they may make an announcement within the next few years, either an upper limit or a real measurement.
The one you have heard about is this one.
And this is the thing that I don't know if you remember a couple of years ago, there was something called the bicep experiment,
which was a experiment that looked at the polarisation of the cosmic background radiation.
And that experiment thought they had been seeing in fact, there was a huge announcement that they had been seeing gravitational waves.
And the way they saw them is by the polarisation, having certain pull that this is the electric field polarisation in the cosmic background radiation,
having certain patterns, in fact a pattern that looks like a spiral,
like it looks like a spiral tube pattern, you know, a circle like this and would occur if,
for example, they were gravitational waves that had periods of the age of the universe.
They would you would they would see that and they would see it in the pattern of the polarisation,
the electric field polarisation of the cosmic background radiation.
And they thought they had seen it.
And what they had seen instead was some dust in our own in our own galaxy, which was causing that, that kind of a pattern.
Now, that made sort of bad news for everybody and people got mad at them.
But I think that that's going to be one of the most stunning experiments is still being done here.
And a lot of groups have joined that search for primaeval gravitational waves that come from the moment of the sort of grand idea.
It comes from the moment that the universe was created.
It comes from 10 to -30 seconds after the universe suddenly popped out of the vacuum, sort of a completely nutty idea.
But that's where that gravitational radiation might come from. And and so that is a huge stakes game.
And this thing is a wonderful thing to look for. And I hope they find it.
So thank you.