In Our Time - Gravitational Waves

Episode Date: May 17, 2007

Melvyn Bragg and guests discuss mysterious phenomena called Gravitational Waves in contemporary physics. The rather un-poetically named star SN 2006gy is roughly 150 times the size of our sun. Last week it went supernova, creating the brightest stellar explosion ever recorded. But among the vast swathes of dust, gas and visible matter ejected into space, perhaps the most significant consequences were invisible – emanating out from the star like the ripples from a pebble thrown into a pond. They are called Gravitational Waves, predicted by Einstein and much discussed since, their existence has never actually been proved but now scientists may be on the verge of measuring them directly. To do so would give us a whole new way of seeing the cosmos. But what are gravitational waves, why are scientists trying to measure them and, if they succeed, what would a gravitational picture of the universe look like?With Jim Al-Khalili, Professor of Physics at the University of Surrey; Carolin Crawford, Royal Society Research Fellow at the Institute of Astronomy, Cambridge; Sheila Rowan, Professor in Experimental Physics in the Department of Physics and Astronomy at the University of Glasgow

This BBC podcast is supported by ads outside the UK. Thanks for downloading the In Our Time podcast. For more details about In Our Time and for our terms of use, please go to BBC.co.com.uk forward slash radio 4. I hope you enjoy the programme. Hello, the rather unpoetically named Star, SN-2006GY, is roughly 150 times the size of our sun. On Monday of last week, it went supernova,

creating the biggest stellar explosion ever recorded. But among the vast swathes of dust, gas and visible matter ejected into space, perhaps the most significant consequences were invisible, emanating out from the star like the ripples from a pebble thrown into a pond. These are called gravitational waves. They run through the fabric of spacetime itself, and having been predicted by Einstein nearly 100 years ago, we may be on the verge of proving they exist.

But what are gravitational waves? Why are scientists trying to measure them, and if they succeed, what would a gravitational picture of the universe, look like. With me to discuss gravitational waves are Jim Alcalili, Professor of Physics at the University of Surrey, Carolyn Crawford, Royal Society Rehears Fellow at the Institute of Astronomy, and Sheila Rowan, Professor in Experimental Physics

and the Department of Physics and Astronomy at the University of Glasgow. Jim Alcalili, gravitational waves are first predicted by Einstein's theory of general relativity, published in 1916, following up on his paper on Special Relativity, published in 1905. Can you just tell us how those two papers affected the then-going notion of gravity. Yes, well, in Einstein's special theory of relativity, that's the E-E-E-E-E-E-squared theory.

So that's the one that came out first in which the notion of space and time itself were changed. You know, the idea that space is absolute, it's the stage on which everything happens, and that time is there's a cosmic clock ticking by at a constant rate everywhere. That's the Newtonian view of our universe that was overthrown in Einstein's special thing.

of relativity, but it took 10 more years before he could incorporate the idea of gravity into his theory of relativity. Now, gravity is also something that Newton had something to say about, but Newton's view of gravity was of this sort of magical, invisible force that pulls all objects together. So the reason we stick to the earth, the reason the earth goes around the sun and so on. And Newton's picture of gravity is as a force that acts, instant instantaneously between objects. What Einstein did in his general theory of relativity was to explain gravity not as a force,

but as something that happens to space and time themselves, sort of curvature of space and time. Now, these are words, and we can say them, and a lot of people all have heard the notion of curvature of space and time, but it's a really, really complicated concept to try and imagine. You know, space is three dimensions. We know we live in three dimensions of space. Time Einstein tells us is the fourth dimension.

And so you have a four-dimensional space time, which we can't imagine because our brains are only three-dimensional. And then you think, well, four-dimensional space time, gravity causes it to bend, to curve. And you can't have a picture of something that bends in curves. After all, you know, we just haven't got the facilities to think in these higher dimensions. So it's a very abstract mathematical idea.

But it's also a very beautiful and simple theory. Einstein's general theory of relativity is one of the most accurate and beautiful mathematical theories ever devised. Is there a sense in which he refined Newton or erased Newton? Well, in terms of the picture he gave us of gravity, it was very different. In terms of the predictions as to how strong the gravitational force is between any two objects, it was refining Newton. So, for instance, when NASA send rockets to the moon

and out into the solar system, they don't need to worry about Einstein's refining of Newton's law of gravitation. They go because of Newton. They go because of Newton, and Newton's perfectly fine, for all intents and purposes. Where Newton's theory breaks down is when gravity gets very, very strong, and that's where these new predictions, like gravitational waves, start to come into play.

Right. So you think that's enough to say about the change in the idea of gravity before we move on towards these gravitational waves? Well, we can come back to clarify these. Okay, okay. Carolyn Crawford, how did these ideas then cause him to predict gravitational waves? Well, you have a mass in space and that warps, as Jim says, it warps the space and time around it, and the force of gravity is due to this curvature. That's fine if you've got a mass still in space, you've got this fixed, distortion of the space around it.

But if that mass begins to move, the gravitational field around it has got to change. That means the space time, the shape of space time, has to adapt and evolve to take into account this new gravitational field, to take into account the motion of this mass. And so when a mass moves... We're talking about a huge object in space, any huge object or a cluster of objects. Well, I mean, just pick a huge object in space,

just nice mass in space. If that moves, is going to set up a disturbance in space time. And so what these waves are, they're almost like a signal that carries out the information to the rest of the universe that the gravitational field around this object has changed. So they're actually, they propagate outwards from the motion of this mass at the speed of light.

The thing that's very difficult to get your head round is the idea that these are not, we call them waves, but they're not like waves as we're used to them. We think of waves like light waves, radio waves that travel through space across the universe. The difference is that gravitational waves are actually distortions of the space time itself. So they actually travel through and distort the space and time. So it's quite a strange concept.

It is. Can you say it again in another form of words so we can absorb it a second time? Okay, I'll try. One analogy is to other theories we have, for example, how light and radio waves are produced. they're produced by electric charges in motion. That comes out of, maybe you've heard of Maxwell's theories of electricity and magnetism. It's a beautiful mathematical solution.

It predicts the existence of these waves. Now this is a similar thing. You have a similar kind of theory. It predicts from a mass in motion, you're going to set up these ripples in space time propagating out. So, I mean, the key things are, I mean, this is a very simple glossing over what's happening. it's not just the mass has to be in motion. It also has to be accelerating.

That means there has to be a change in either the rate of motion or the direction of motion. And this has all got to happen in an asymmetric fashion. So we're now kind of layering up lots of different criteria to produce gravitational waves. What kind of things give off these gravitational waves and why? Well, the kind of things that give off these gravitational waves, as I say it's got to be something in motion,

something where that motion is changing, and it's got to be heavy in asymmetric fashion. So if you just have a mass spinning or a spinning disk, that's not enough. It's still symmetric. Lingerly symmetric, spherically symmetric, is still going to not give off any gravitational waves. Even a mass just moving through space and time

isn't going to give off gravitational waves. However, if you have a kind of situation, imagine something is shaped like a bone or a dumbbell. If it spins along its vertical axis, you've got a symmetric situation. That's not going to give off gravitational waves. But imagine now it tumbles end over end. You've got a different kind of motion, and that's the kind of motion that sets up gravitational waves.

So go back to space. We get gravitational waves from motions on Earth, but they are insignificant. They're absolutely tiny. The amount of radiation you get goes up very strongly as the mass and the speed that you're travelling at. So to get any kind of significant gravitational wave, that's where you have to go to these astrophysical phenomena.

You need an enormous mass, travelling at a speed that's nearly relativistic. And it's got to be doing this in an asymmetric fashion. So we get black holes or supernovae and binary stars. When we get... Giving up gravitational waves. Can I just go to Sheila for a moment? I'm sure we'll come back to this.

Einstein predicted them in his theory, gravitational waves, as I understand it. But what was the first few things? of evidence that gravitational waves might actually exist? Well, we do in fact have strong indirect evidence that they exist and that came again from an astrophysical observation. And in the 70s, 1974 I think, there was a pair of scientists, Halson Taylor,

who were looking at pulsars, a particular kind of astrophysical source where we have something called a neutron star, which is a particular kind of star, which is in some ways like a giant atomic nucleus that's made up of the same kind of particles, neutrons that you find in the middle of atoms, but on a huge scale.

So, and a neutron star, under some circumstances, it can spin, if you've got a spinning neutron star, it kind of beamed radiation that comes out of its ends. And it's a bit like a beacon, a bit like a lighthouse. as it spins, that radiation can be beamed towards us on Earth and we see pulses of radiation. And so they were looking at pulsars and they noticed a particular pulsar

that's in what we call a binary system. And this means that there were two stars orbiting round one another, orbiting round a common centre. And one of these was a pulsar. And over a long period of time, over months and years, what they did really was from the measurements they were making, they were able to watch the evolution of the orbit. In other words, they were measuring how long it took these two stars

to orbit round one another. And they did that over a long period of time. And they could see that the stars were getting closer and closer together. They were losing energy. And the general relativity allows us to predict how fast that orbit should be to be. and what the change in the orbit should look like. And that prediction of general relativity

includes the fact that this binary system should be radiating away energy in the form of gravitational waves. And when they compared the observations they were making with the predictions of general relativity, they agreed beautifully. And that agreement comes about in part because energy is being lost due to gravitational waves. But we still haven't direct evidence.

But can you, what would happen, let's try to bring it down to Earth, at least to this studio. What would happen if a gravitational wave we're passing through this room right now? And we hope that they are. We believe that they are. And as Caroline said, the distort space time. What does that mean to us? Well, if we look down sort of at a coffee cup sitting on the desk here, the top of the coffee cups round. If a gravitational wave comes from above us, goes through the studio, and passes through our coffee cup,

what it will do to the shape of the coffee cup is it'll change the shape. So if the top starts as a circle, as a gravitational wave passes through, that circle will become distorted into a rugby ball shape. And then, as the gravitational wave cycles through, it'll go back to being a circle, and then it'll become a rugby ball shape in the other direction. and cyclically that that circle will be squished, stretched and compressed. And as we sit here, we're all being stretched and compressed just slightly as gravitational waves pass through us.

So it's changing the shape of objects. Jim, do you want to come in for a moment? Yeah. The thing is that there are these two predictions of how we would detect gravity, gravitational waves. The clincher would be, as Sheila says, to actually detect distortions, stretching and squeezing of space time. Now, again, we say space time, which is pedantically correct, but effectively what we're talking about is...

I can't by the only person who's spinning around trying to sort of see what space are in. So I will, with apologies to fellow scientists who like to use the term space time, I will just use the word space. Space gets squeezed and stretched. As Sheila explained, if a gravity wave passes through any region of space, in one direction, lengths, contrasting. and get shorter, and in the other direction at right angles to it, they will get longer.

So it's like a distortion of space itself. So this has happened minimally now, but Carolyn, before we look at the existence of gravitation ways, and we're going very steadily towards that, and how they might be proved. Could you explain, as an astronomer, why the proof of their existence, although we have extremely well set out indirect proof from Sheila,

why that will be a significant breakthrough? Well, the foremost reason, of course, is that this is a prediction from Einstein's theory of general relativity that is yet to be verified. And so at the simplest fundamental level, it's verifying the whole theory of general relativity and our understanding of it. And the kind of objects, Sheila described,

two neutron stars going around each other. Imagine you could receive the signal from two black holes going around each other. They're travelling at enormous speeds in a huge gravitational field, looking at how they react to each other, the gravitational signal coming up from there. You get these really acute tests of our theories of general relativity.

But astronomically speaking, you can take it further than that. I mean, gravity is everywhere in the universe. Everything we see is ordered about controlled by gravity. But astronomers have to rely on looking at the light that's emitted by objects whose motions are controlled by gravity. Imagine we could detect gravitational waves. We would be actually observing the gravity firsthand. Instead of inferring what's happening secondhand

from the light of these objects being controlled by gravity, we start to get signals from gravity itself, and that is quite a shift in how we might view and observe the universe. So we're getting signals from the 95% of the universe that we can't see, which might be the... You did a grimace that. I've got it wrong, haven't I? Well, no. Sharp and ticker.

breath from all three games. Yes, okay. Is that your head? Just sort that out. It'll change the direction. I mean, of course. I mean, we, again, it's, we only see the light from a tiny fraction of the whole universe. There's this, as you say, right, there's this huge fraction which is invisible, yet still

produces gravity. And yes, so we're going to get gravitational signals from both dark matter and from light that's emitting. So the key things, we're going to be able to observe the behavior of gravity. The other thing that's really nice is that we used to light. Light gets stopped by matter. It gets scattered by matter. As it travels across space, the light waves get modified.

They get changed. Gravitational waves pass through matter completely unscathed. I mean, as Sheila said, they're going right through us now, right through the earth now. That means that if we've got some circumstance, imagine, you mentioned a supernova. This happens when a star at the end of its life, a giant star, after a few million years, it can no longer hold itself up against gravity and it dies in this spectacular explosion, throws off the outer layers of the star,

the centre of the star implodes to form a neutron star, a black hole. But you can imagine all of this is really sheathed from our view, is obscured from our view from the light. But the gravitational wave signals that mark the birth of this neutron star, a black hole, will be able to be directly detected. Sheila, can I bring you it again? You explained very eloquently the indirect evidence and the pulsars and so on.

What about what will the direct... evidence if it is arrived at, which we'll come to later in the program, how people are trying to arrive at it as we speak. What will that bring? The direct evidence means that we'll be, well, we'll see directly the effect of the squishing of space time on objects here on Earth, and it'll let us look back, as Caroline said, to understand about the sources, direct viewing of sources in a gravitational picture. and so it will bring with us information about things like black holes in particular are a very interesting source. If a supernova does happen and a star collapses to form a black hole,

and people now really believe black holes do exist. I think there's really no, you know, the controversy about them is gone. People believe they exist, but they're still very exotic objects and difficult to view. Because by definition, a black hole is something that's got so much gravitational pull, that nothing can escape from it. So we can't get information about the black hole itself. Now we can do observations using the techniques we have of stuff round about the black hole

so we can see light in x-rays from gas around about the black hole, but we can't probe the black hole itself. But the gravitational picture may let us see really what the black hole itself is doing. If we've got two black holes, and again we believe this is something that could happen if we have two black holes in a binary system

again it's two black holes orbiting round one another losing energy that come in and coalesce at the edge of the black hole and the sort of boundary in space time beyond which nothing can escape when those two black holes coalesce there's disturbances of that boundary gravitational disturbances

and they should cause direct gravitational signals that we could detect here on Earth and see kind of the edges of a black hole in a real gravitational picture. Jim? I think it's also important to stress, before we move on to how we detect gravitational waves, just how difficult it would be. Einstein published his General Theory of Relativity in early 1916,

and it was only a few months later in the same year that he published a paper where he predicted that gravitational waves should exist. This was a prediction purely from the mathematics of the equations of his theory. In fact, he got the answer wrong, apparently, by a factor of two, and the English astronomer Arthur Eddington had to point it out to him. Eddington was also an expert on general relativity. In fact, I think he was once asked by a colleague that, is it true, that he was one of only three people who actually understood general relativity,

and he went quiet, and the colleague said, oh, don't be modest. He said, no, I'm trying to think who the third person might be. But since that time, 1916, and to now, we're only now designing the experiments to detect gravity waves, as we mentioned before, a gravity wave would cause space length to shrink, contract by a very, very small amount. A typical gravity wave, bearing in mind that universe is so big, and these objects like coalescing black holes and supernovae are so far away,

the gravity waves that reach us typically will be so weak that a metre length of space would change in length by about a millionth of the diameter of an atomic nucleus. So we're down into the sort of just a vibra-random brownian motion, vibrations of atoms within a mass, a solid object, will completely swamp the effects that gravity waves will have on that object. So it's incredibly difficult to actually pick up these weak signals. How do they compare then with other kinds of waves like electromagnetic waves?

Oh, electromagnetics are many, many trillions of times more powerful than a gravitational wave. I mean, to give another example, this indirect detection of gravity waves through this whole tailor binary system of neutron stars, it was the energy given off by the gravity waves rather than the waves themselves that were picked up.

Well, the Earth going around the sun would also give out gravity waves because, again, it's a disturbance of mass in space. The energy given off by, the power given off by the Earth's sun system, is equivalent to something like half a dozen light bulbs. So, you know, half a dozen 60 watt light bulbs. So imagine how massive and how violent these objects have to be out in space

if they're so many light years away from Earth that they'd have to give off waves that we can pick up. The other thing, of course, is that these, as Jim describes, they're going to be very violent cataclysms to produce detectable gravitational waves. They also have to happen relatively near to us. of course this is space, by relatively, I mean, within so many thousands or millions of light years. A lot of the uncertainty about gravitational waves is not only what sources produce them, but how frequently these occurrences happen.

We've talked about neutron styles or binary black holes coalescing, orbiting around each other. How often do they merge together? Is it like one a century within our galaxy, one every 10 years? There are big uncertainties like that, let alone all the other really challenging. difficulties for detection. Could the gravitational waves shed any new light, new light on the origin of the universe? Oh, that's very interesting.

Go back to the origin of the universe. There is an idea that after the Big Bang, there was this period of exponential growth within the universe called inflation. And there are ideas that there should be a background, a diffuse background everywhere of gravitational waves produced within this inflationary period. and this should have produced some distortions back then

that could be visible on this, what's called, the last scattering surface. This is a cosmic microwave background. And it is hoped that some future experiments like Plank will actually start to pick up some of these signals from this very early universe. Can I come to you, Shudder, you were on a program we did a couple of years ago,

on the graviton, which as I understand is the particle of gravity. How does the graviton relate to gravitational waves? Right. The idea that there should be a particle associated with gravitational waves is an extrapolation from what we know about other kinds of forces like the electromagnetic force. There are four fundamental forces of which gravity is one. The other forces, we believe we have good models for the fact that there's a particle which helps transmit the force associated with the force. So that's true for the electromagnetic waves.

There should be particles associated with them, virtual particles. So we, and that's, people believe then gravity is a force. It's likely that it should be a particle associated with it. That's how it can be transmitted. The difference is that for the other forces, we can apply quantum mechanics to them. We have a tool for dealing with these forces that involves being able to apply them over small scales and using a specific mathematical technique to do that.

Gravity, described beautifully by general relativity, is different. We can't apply the same tools to it as we can to the other forces. So whilst we feel there should be a particle associated with gravity being transmitted and gravitational waves being transmitted, we don't have the tools really yet to describe that. That particle would be the graviton. So despite the fact it has a name,

we don't have a good model for how that would fit into the theory. And we can't be certain that it exists. We can't, I think. So we're talking, a lot of this is bending the imagination at 925 time, no, nine space time. And so we're talking very seriously about things that might not exist relating to things that we can't find you, the gravitational way. I think we should distinguish between the graviton and gravitational waves.

The graviton is, I think, a step too far for this particular discussion in the morning. Gravitational waves, on the other hand, I think we have such a good theory that predicts them, such good indirect evidence that they exist. There's no other way to explain the observations that have been made, that we really do have confidence that the gravitational waves are there for us to see. It's just that it's a very, very hard experimental task. to do it. It's a great challenge. Gene Archelaine, is this part of the bigger problem in physics trying to reconcile general relativity to quantum physics?

In a sense it is. I mean, gravitational waves are yet another way of saying Einstein's general theory of relativity is correct. And ever since he's published his first paper, there have been experimental tests of general relativity, and they've all come through with flying colors. gravitational waves, along with one or two other predictions of general relativity, are still waiting to be nailed. But we don't have any doubt that they will sooner or later. But in terms of reconciling all the theories of physics together and all the forces of nature together,

the graviton certainly is a prediction of that, but as Sheila says, that's something for the future. But we are still testing general relativity to see whether, in fact, it is 100% correct. It may need modifying if we're ever going to merge it in with the other theories that describe the other forces such as the theories of quantum physics. Sure.

Indeed, gravitational waves, although we've talked about them being a prediction of general relativity, in fact, it's really special relativity that tells us there's got to be something like gravitational waves. And that what special relativity tells us is that you can't transmit information faster than the speed of light. And that's any kind of information. You can't send a signal to your friend on the other side of the universe

in any way with that information travelling faster than the speed of light. And that's true for gravitational information. So if a star somewhere on the far side of the universe explodes and there's a big change in its mass distribution, we can't know about that instantaneously. It's got to take time for that information to get to us. In other words, there's got to be propagating, traveling gravitational information. gravitational waves.

And so even without general relativity, some form of gravitational radiation has to exist to be special relativity. And there's a nice example of that. We talk about that the light from the sun takes eight minutes to reach the earth. So if the sun were to suddenly cease to exist, it would take us eight minutes to realize it

before the world, the sky goes dark. It would also take eight minutes for the earth to realize that there's no longer any gravitational pull and that it can just float away off into space. So the effects of gravity from the sun, we believe, and the lights coming from the sun travel both at the same speed. Okay, let's devote the rest of the program to the attempt to detect these gravitational waves, which, apart from anything else from what I've read for this program,

involves a most extraordinary feat of technology, which I hope you'll talk about in some detail. But let's just give it a context. We've talked about gravity, we've talked about gravitational waves, waves. Carolyn, Concord, what are the problems you're facing in trying to detect it, trying to detect it, what is basically a ripple in the fabric of space time,

which is, as Jim pointed out, kindly at the beginning, we can't imagine. Well, the problem is, and again, as Jim said, over a metre's length, this distortion is going to be this tiny fraction, somewhere like a millionth the size of an atomic nucleus. and that is just such a tiny signal you're looking for. And that is really the main challenge

because, as you say, there's brownie in motion. There's all kinds of other noise that can set up vibrations, can set up kind of, that can mask this signal. In terms of how we detect them, the kind of method now that is used, there are a couple of observatories in the states, where you have an L shape. And what you're looking for,

is for one branch of the L to be stretched at the same time as the other branch is squeezed. Can I just hold on a second? Before we go, and we've got plenty of time for this, actually. I just want to know, to listeners to know, what are, you mentioned one of the problems that can, it's so tiny and other things can cause similar ripples and this. Could you just develop better a little bit, just so we've got a real idea why it's so difficult?

You've said it, I'd just like you to extend that a bit if you could. Maybe I can say a few words about that. Gravity is a very weak force. It doesn't seem like that to us, because it's what holds us on the earth. But in fact, it's very weak. So these gravitational effects we're looking for are very, very weak. I think if we can go back actually to your introduction at the very beginning,

your analogy of throwing a stone into a pond and seeing the ripples come out isn't a bad one. And if we imagine our universe, in fact, is like a big flat piece of rubber on which we put a mass, we plonk the sun. That rubber then curves so that if we brought along

another object and tried to sit it on the rubber sheet, it would roll down that curve towards the first object down that curve on the rubber sheet. That curve on the rubber sheet we think of as being gravity and it sits there static force.

Then if that star in the middle our sun or another star wobbles a bit, it changes its position its mass, it moves, it sends out ripples across the rubber sheet. Those are the gravitational waves we're trying to detect. And the trouble is those ripples are tiny. So if we look at two points on the rubber sheet, they will be stretched and compressed.

We said that was the effect of the gravitational waves, but it's a tiny, tiny amount. And what we do, as Carolyn mentioned, is we literally on Earth plonk down two masses. And they are masses, the pieces of glass, they're mirrors, They're a reasonable size, about six kilograms or so. So it's kind of two chunks of glass. And we try and measure very accurately their positions

and measure the changes in their positions as a gravitational wave passes through them. And as Jim tried to give us an idea of how small the change in their position is, it's absolutely tiny. The mirrors, these are pieces of glass with coatings on the front to make mirrors. The changes in the positions of those mirrors, as he said, is much, much smaller than the size of an atomic nucleus, tiny, tiny effects.

So as they sit there, you can imagine, we put them on the ground, we try and measure their position. We need a very accurate ruler to do that, is the first thing. And the ruler that we use is actually the wavelength of light. We take light from a laser. So waves of light from a laser come along. We put in a what's called a beam splitter, which splits that light into two,

and those waves of light go out, bounce off our mirrors, come back again and add up. And how they add up, they can either add up to give us a bright spot, which we would literally see, or a dark spot. And whether it's bright or dark depends on how far the light waves have travelled when they hit these mirrors and came back again. So the brightness of the spot that we see is giving us information about how far the lights travelled and what the positions of these mirrors are.

Do you like to take that up, Caroline? Yeah, well, again, go back to this L shape. As Sheila says, you have these lasers travelling along each stretch of the L, and you're looking for exactly the signal that it's changed in one, it's stretched in one side of the owl, and it's squeezed in the other side of the L. And you get this by measuring to this fantastic accuracy, the length, the path that the light has travelled in each branch of the L.

Jim, is there a history? Jim, Mark Lillian, is there a history to these kinds of experiments? Yes. Can you give us some context here? It's very interesting that this idea actually goes back to the second half of the 19th century to one of the most famous experiments in physics. In fact, it was the experiment that got Einstein interested as a young boy in notions of space and time from the very start.

The idea that light waves interfere and cause these patterns and fringes was known long before, the English physicist Thomas. Young showed this back in the first few years of the 19th century. But two American physicists around about the 1870s called Mickelson and Morley developed an experiment where they were looking for a medium in space called the ether that most scientists at the time believed was the medium that carried light waves. So in the same way that we need air to carry sound waves and the reason why in space nobody hears you scream,

and when you watch Hollywood films and seeing spaceships explode there shouldn't be any sound which isn't really very good for the film but if there's a vacuum, sound doesn't carry but light waves clearly do travel through a vacuum as light reaches us from stars and the sun

so physicists in the 19th century believed there had to be something that carried light waves and Mickleton and Morley designed this experiment using this device which we now call an interferometer where two light beams, travel in two directions at right angles, covering two different paths that can be very carefully controlled, bounce off mirrors, come back and recombine and interfere.

And the way that interference pattern changes tells us how far each separate light beam has travelled there and back. So any changes in that path length, if you can control everything else, tells you whether the path has changed. And it's developed now to the latest one, Sheila-Roe, and a plan for a new experimental setup called the laser interferometer space antenna.

What is that going to do that hasn't been able to achieve before now? Right. Well, there are two classes of these experiments. Experiments done on the ground and experiments done in space. And before we get to laser interferonter space antenna, that's the space version of these experiments. I think I should probably just say a little bit about the ground experiments, which explains partly why we're going to put one in space. The experiments on the ground, as you just heard, measure the positions of mirrors, okay, very accurately.

But you can imagine if we just took our mirror and sat it on the ground, tried to detect its motion due to gravitational waves, would be completely swamped by the fact the ground's moving. You know, a truck drives past, shakes the ground, the mirrors move far more than any gravitational wave would make them move. So we carefully suspend them as mirrors. That turns out to be a noise source that we can relatively easily get around just by hanging the mirrors as pears.

pendulums, and that pendulum acts as a mechanical filter to get rid of seismic noise. So that we can get around. There's also the thermal noise of these mirrors shaking. The individual atoms and molecules have some temperature. That temperature causes them to shake slightly. So that's a noise source that's harder to get around. And there are various noise sources like that, which we work very hard on reducing, to the point where we have built detectors on the ground,

that can measure these tiny, tiny, tiny displacements that are just about the level we would expect to see gravitational waves at certain frequencies. At the moment, between about 50 hertz or so, up to a kilohertz or so. So these mirrors would be shaking, say, between 50 and a couple of thousand times a second due to gravitational waves. That's what we're trying to see.

And there are certain sources, some of the, astrophysical sources we talked about that should produce signals at those frequencies. The supernova for instance, the coalescence, the coming together of these binary stars, we hope to be able to see those on the ground. But at very low frequencies,

so sources that produce very slow gravitational changes, we won't be able to see on the ground. And for that we need to put a detector in space because there's a particular noise source on the ground, just called gravity gradient noise. People slowly walking past these mirrors

exert a straight gravitational pull on them. But that only happens, and that's bigger than any gravity wave effect on the ground, but that only happens slowly. You only walk slowly past the mirror. You don't run past the mirror a thousand times a second. So for those low frequency sources, to get away from that noise source,

we have to put a detector in space. Do you want to come in and give, listen, one of you, some idea of the size of these? Carlin. Well, the idea behind... Let's go into space with these things. Yeah, we've got time to say... Yeah, go into space with Lisa.

And the idea is that you have three spacecraft that are the corners of an equilateral triangle. And they're going to trail around... They're five million miles apart, is that right? Yeah, I was going to tell you that, yes. Oh, sorry, yeah. Yeah, the ground experiments, these that are currently running, the length of the L shapes is about four kilometres.

The advantage of putting them in space is that these space spacecraft at each side of this triangle, they're all going to be five million kilometers apart. So you have the amount of signal that you detect goes up over the longer the distance you measure it over, and so that increases the sensitivity, as well as increasing the range of the frequency that we can detect these waves from, as Sheila said.

Can you just give us some idea of the precision involved, the technology involved, because from what I've read about it just seems, again, almost as mind-based, boggling as a theory is really. It's remarkable that we've known that gravitational waves should be out there and the technology is advancing all the time. But even these experiments, they've been, they've taken so many years to design and build. I mean, Lisa won't be ready for another 10 years or results for another 10 years or so, maybe earlier. Well, it depends whether it's funded. It's not even

certain yet. It's up for competition against other space missions. Certainly from the, it's being funded by the American and the European space agencies. The Europeans have given some money to it, but the Americans have still decide where they're going to fund it. There is a body of opinion that so much money, it's got massively expensive, may not be best spent detecting things that may not actually exist. As I say, I think we're pretty sure that gravitational waves do exist. There isn't a great controversy over that anymore.

They're hard to detect, but I think there's no real genuine controversy anymore moreover whether a form of gravitational radiation exists. We do believe they exist. Of course there's, you know, playing general's advocate. It's interesting using words like believe here. I mean, there's another program. There's no direct detection yet, but there's very strong indirect evidence.

And all our theories tell us that some form of gravitational radiation has to exist. Or it's going to cause us great problems with our current theories for how we believe the universe behaves. The universe should exist. if it tells us that general relativity is wrong, that's going to be an enormous revolution in physics. So if we can show that general relativity is wrong, then that's going to be huge.

I mean, that's the crucial point, because one could also argue that if we are so sure that gravitational waves exist, why should we spending so many hundreds of millions of pounds to design an experiment that's just going to confirm what we already know? And the point is that we are trying to test general relativity and push it to the limits

and see if it really is correct and needs modifying. So along with all these new theories to try and unify the forces of nature, some of them are expensive, some of them worthwhile. They're part of our inheritance. And gravity is the least understood force of all the forces. It's the most significant force that governs the behavior of our universe and yet the least understood in some ways.

Just over the last few years, the last five to ten, years the significance of dark matter and dark energy, the fact that our universe is expanding much faster than we can possibly understand. Gravity somehow has got to play a role in that and we really don't understand how. So experiments to understand gravity better and general relativity better are hugely significant for how we understand our universe. Well, thank you very much for letting me accompany you through the last 44 minutes. I've enjoyed it and I think I've I've understood a bit of it.

I hope I'll remember enough of it next week. But thank you very much as Jim Archelieu, Carolyn Crawford and Sheila Rowan. Next week, it's the Siege of Orleans 1428, Henry of 6th, and Joan of Arc. Thanks for listening. We hope you've enjoyed this Radio 4 podcast. You can find hundreds of other programmes about history, science and philosophy at BBC.com.com.com.