In Our Time - The Graviton

Episode Date: November 24, 2005

Melvyn Bragg and guests discuss the search for the Graviton particle. Albert Einstein said "I know why there are so many people who love chopping wood. In this activity one immediately sees the results". Einstein spent the last thirty years of his life trying to find a theory that would unify electromagnetism with gravity, but success eluded him. The search is still on for a unifying theory of gravitational force and hopes are pinned on the location of the graviton - a hypothetical elementary particle that transmits the force of gravity. But the graviton is proving hard to find. Indeed, the Large Hadron Collider at CERN still won't allow us to detect gravitons per se, but might be able to prove their existence in other ways. The idea of the graviton particle first emerged in the middle of the 20th century, when the notion that particles as mediators of force was taken seriously. Physicists believed that it could be applicable to gravity and by the late 20th century the hunt was truly on for the ultimate theory, a theory of quantum gravity. So why is the search for the graviton the major goal of theoretical physics? How will the measurement of gravitation waves help prove its existence? And how might the graviton unite the seemingly incompatible theories of general relativity and quantum mechanics? With Roger Cashmore, Former Research Director at CERN and Principal of Brasenose College, Oxford; Jim Al-Khalili, Professor of Physics at the University of Surrey; Sheila Rowan, Reader in 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 four. I hope you enjoy the program. Hello, Albert Einstein said, I know why there are so many people who love chopping wood. In this activity, one immediately sees the results.

Einstein spent the last 30 years of his life trying to find a theory that will unify electromagnetism with gravity. but results eluded him. The search is still on for a unifying theory of gravitational force and hopes are pinned on the location of the graviton, a hypothetical elementary particle that transmits the force of gravity. But the graviton is proving hard to find. Indeed, the next big research project,

which involves the largest earth-based laboratory in the world, a circular ring which goes underground for about 27 kilometres and spans Switzerland, France and Germany, still won't allow us to detect gravitons per se, but might be able to prove their existence in other ways. So why is the search for the Graviton the major goal of theoretical physics? How will the measurement of Graviton waves

help prove its existence? And how might the Graviton unite the seemingly incompatible theories of general relativity and quantum mechanics? With me to discuss the Graviton, Roger Cashmore, former director at CERN and Principal of Brazeno's College, Oxford,

Jim Alcalini, Professor of Physics at the University of Surrey, and Sheila Rowan, reader in physics in the Department of Physics and Astronomy at the University of Glasgow? Roger Cashmore, Now, Graviton is a hypothetical elementary particle that we think mediates gravity. What does that mean?

And can you explain to us why we think it might exist? Well, perhaps I should begin by saying that, in my view, that gravity is one of the least understood of the forces of nature that we have at the moment. And so a lot of effort goes into trying to understand that gravity. Now, we all know the effects of gravity. We feel we're pulled down onto the earth. We see apples as Newton did fall. And we know gravity is there.

We know other forces of nature are there as well, which we've been successful at probing the electromagnetic interactions, the weak interactions, the strong interactions. And what we know about all of those interactions is that there are particles that mediate them. And they carry the force around, from one particle to another. Now, making the analogy with gravity, where you have a gravitational field, a gravitational attraction,

you're looking also to see whether there is a similar particle that mediates gravity that carries a gravitational interaction from one particle to another particle. That is the graviton. So, if we can find a graviton, then we will be able to gain, say, we've made an understanding of gravitational fields and the attraction, the gravitational force, and how that can be perhaps put together with quantum mechanics, as you indicated earlier.

So it would be the mediator of this gravitational attraction, carrying the force from one particle to another. Now, the problem with gravity is, surprisingly, is it's very, very weak. It's the weakest of the forces of nature that we know all about. It's, of course, the one that has the biggest effect on us as people on the surface of the earth, and as we look up into the cosmos, it's gravity that is doing a lot of the work that controls how the universe evolves and how things, how stars are formed, etc. So it's a surprising dilemma is that it's very, very weak, but it's the one thing that we know and see around us very strongly.

So what is this dichotomy here? When you're weak, the illustration given by one of you in the notes that I got was it's weak. For instance, you put a little magnet onto a fridge and that defies gravity. It doesn't fall to Earthers. And so in that sense, it is weak. On the other hand, as you've just said, it's a great controlling force here and out there. And why does that give you a dilemma? And why if you can find the mediating particle for electromagnets and the weak nuclear force

and strong nuclear force. What's so difficult then about finding the gravity on it? Well, to compare with the magnet on the fridge, let me take you one step further. Inside an atom, you have a hydrogen atom, you have an electron that goes around a proton in the nucleus, and that's held together by this very strong electric force. The gravitational force of the electron with the proton in an atom

is millions and millions and millions and millions, millions of times smaller. Therefore, the way gravity gets its strength is because every piece of matter adds up together with this very small force to give you actually in the aggregate a very strong force. But if you look on a small scale, then the electromagnetic interactions, the electric forces are much more dominant. Now, the difficulty with getting to the graviton is that the graviton is this thing which interacts with another piece of material.

It's the strength of that interaction, which makes it difficult to see bounce off it or to be it absorbed by it, and that is this very, very weak interaction. Why is it so important, Jim Alkalili, to find the graviton? Why is it so important? Well, Roger mentioned that

the force of gravity is the least understood the forces of nature, but the two greatest physicists of all time, Isaac Newton and Albert Einstein, both gave a description of the force of gravity, and one would think that we should by now have it sewn up. So it starts with Newton. Can you tell us how it starts with Newton? Well, for most people, Newton was the person who explained the force of gravity.

He had his famous law of gravitation, the inverse square law, which developed in the 17th century. And for most intents and purposes, that's a perfectly good description. but Newton's view of gravity was one of some sort of mystical, invisible force that's like an invisible rubber band that pulls all objects in the universe together. What Einstein did was go beyond this. He didn't like this idea of what he called action at a distance. He described gravity in his general theory of relativity as the curvature of space and time,

which is much, much more difficult to create. comprehend for the non-scientist. Can you have a go to comprehend it out loud for us now? Okay. Well, Einstein basically described gravity not as this invisible force between objects, but rather as, in terms of geometry,

pure geometry, the shape of space itself changes in the presence of mass. So an object like a star or a planet or even a person causes all the space in their vicinity to warp in a way

that we can't comprehend because, you know, what direction does it change into, does it bend into? It's an idea that we can understand mathematically, but very difficult to visualize, but it gave a deeper, more satisfying description of the effects of mass on space around them. And so Einstein seemed to have gone beyond Newton. But what he did... Did that give us answers, or did it give us more questions? No, it certainly gave us a lot of answers. It turns out that, Newton's law of gravity is fine as long as gravity isn't very strong. So here on Earth, and in sending rockets to the moon and to the planets,

we can use Newton's law pretty well, pretty accurately. Einstein's view of gravity is the one we have to use when we consider very strong gravitational fields, near a black hole, for instance, or a massive star. And it predicts other effects which Newton's law of gravity never predicted. I mean, something that I'm sure Sheila would mention later, gravity waves, is something that Newton would not have been able to predict. Well, let's mention it now, Sheila.

Can you talk about these gravity waves, these gravitational waves that Einstein predicted, and what are there? Is that taking us towards the search for the graviton? Okay, well, perhaps if we start by taking a little bit further, the geometrical analogy to what gravity actually is, and so to help a bit, if you imagine our universe, first of all, is a big flat rubber sheet. stretched out. It's a nice big flat rubber sheet. Now with no mass sitting on the sheet,

the sheet is just flat. Now so you come along and you put a big spherical mass onto the rubber sheet, the sheet curves, and so you get a dip in the rubber sheet. And if you came along with another object, tried to sit that on the sheet, it would roll down the curve towards the first big piece of mass you put there. And that curvature on the rubber sheet, that curvature in space-time, is really what what we're thinking of is gravity. It's a curvature in space time. So if you then imagine that the mass you've got in the universe,

that mass you've put on the rubber sheet, starts to move, it bounces up and down a little bit on the rubber sheet. What it does then is it sets up ripples on the rubber sheet that propagate out, changes in the curvature of the rubber sheet that propagate out through the sheet. So changes in the curvature of space time propagating out through the universe, these fluctuations and gravity, and that's what we think of as gravitational waves.

And so what did this do to Newton's theory and how did it make the search for the graviton more important? Let's try to stick to that. Okay. Well, as Jim mentioned, in Newton's theory of gravity, the information about the gravitational force and changes in that happened instantaneously. So if a star moves on the other side of the universe,

we'd know about it instantaneously. But what general relativity tells us is that these gravitational waves propagate at finite speed. They propagate at the speed of light. So that information isn't transmitted instantaneously, which helps us a lot because it's a bit of a problem if you believe that information is transmitted instantaneously. So we've got information there about changes in gravity propagating out through space. and we heard earlier that of the other forces that we understand, they are mediated by the exchange of particles. So in the gravitational case, that would be the graviton.

So in some sense, you can see that there might be an analogy here between the production of gravitational waves and say something like the production of electromagnetic waves. So if we think about those for a second, electromagnetic waves, light waves or radio waves are produced by the acceleration of electrons. So if you accelerate an electron, it radiates electromagnetic waves, so light. But we know that light can either behave like a wave

or like a particle, depending on how we look at it, can behave like photons. So again, by analogy, accelerating mass can radiate gravitational. waves, or perhaps we can think about the gravitational waves in some way as having particle-like properties. Gravitons.

But the difficulty is, the graviton, if you want to think about the graviton as a particle, if we can detect gravitational waves, really what we're detecting is large numbers of gravitons. We won't see individual gravitons, but it's kind of equivalent to, when you detect radio waves,

don't detect individual radio photons, but you detect a large number of incoming photons. Well, just to bring this particular section of program to a conclusion before we move on to definitions, can I come back to you, Roger Cashmore, you were in charge of the CERN Accelerator for many years, which is the massive thing that is going on to describe. Can you just tell us, give listeners and myself some idea

of what specifically it is you're looking for when you find a graviton, if you find a graviton, what will you find? Well, what happens at CERN, in the large Hadron Clyde, the machine that's being built there at the moment is you're colliding particles at very, very high energies. And you have a lot of energy in the collisions, the highest we've ever achieved so far. Now, what happens when you have one of these collisions, if we're going to produce a graviton, is you can shake up the space time in that region of the collision. and basically produce an oscillation, which actually in this case will be the graviton, a single particle. So what happens is you shake in the collision, you shake the space time up, and out comes a graviton.

Now, we won't actually see the graviton leaving a track in any of the detectors, but it will take off energy. So what you look for is a collision. You see lots of particles being produced in this collision, but there's a collision. then there's something missing. And that something missing is racing off the universe, and you infer that the graviton has been produced. And you can infer it by looking at the patterns of the particles that are actually produced.

But then there's a gap that's something missing, and that something missing is the graviton. And Jim, to come to the two theories, which seem to explain everything, but don't actually seem to be compatible. the theory of relativity and the theory of quantum mechanics. Now, why are these not compatible, and why is gravity, as it were, the gap in the middle?

Well, I mentioned earlier that Einstein's view of gravity is in terms of geometry, in the shape of space time. Quantum mechanics, which is the other great pillar of 20th century physics, describes the world of the very small. It describes the very building blocks of matter, so it explains how atoms are made up, the particles that make up the atoms, the forces between them. And quantum mechanics is now able and has encompassed and explained three of the four forces of nature.

So the electromagnetic force that Sheila mentioned and the two nuclear forces that really are confined within the nuclei of atoms. We know there's this fourth force of nature, the force of gravity, and we know it's a lot weaker than the others. But what's so frustrating is that, you know, three quarters of the forces of nature have been tamed and explained within quantum mechanics. But quantum mechanics is a theory whose mathematical structure is very, very different from relativity theory. So quantum mechanics is a theory which explains forces as exchange of particles, particles that mediate the forces. I mean, to an outside, it's a really banal question, but why is it such a, why is it such a, bother that there are two different points of view? Well, there are practical reasons

because there are certain phenomena in nature which, to explain, we need to bring to bear both quantum mechanics and general relativity. Now, in most phenomena, quantum mechanics, its domain is the domain of the very small and it explains the subatomic world, whereas general relativity is the theory that explains the cosmos and the universe at large.

So they don't tend to conflict, it doesn't matter that the mathematical structure is different. But there are a certain phenomena where you really want to be able to use both theories. So you want to sort of combine them together in some unified theory. And that's where we still have a problem. We're still searching for this unified theory that would explain all the forces together. Sheila, around, do you want to take that on? So perhaps there are some specific examples where both gravity might be very strong.

want to use general relativity or something that talks about strong gravity. And dimensions are very small, so you might want to use quantum mechanics. And cases, well, the beginning of the universe is one of those, the big bang. What happened right back at the beginning? We believe the universe is expanding. And so if we work backwards, we believe it started off very, very, very small indeed. And to try and understand the physics of what happened then, we'd need to use both quantum mechanics and something that describes strong gravity.

And we just don't quite have the tools to do that without finding a way to combine both these theories. And can you tell us a bit more, Roger Casmar, about boson particles and the part they play in this? Yes, I mean the bosons, the things that Jim and Cher being talking about, the interactions that we observe, the electromagnetic interactions, the weak interaction, the strong interactions, they're all mediated by bosons.

And if you, in fact, want to have a force which is very, very long range, which will go out to the edge of the universe as in the case of gravitation, then you're forced actually into always having them mediated by one of these things called a boson, named after bows, who came up with the idea of these particles in a way, first of all. And they are what we all call, and I'm sorry to do this, they all have a spin, and they have a spin they're rotating in some sense. And these are all spin-one particles that mediate these forces and can give you these long-range effects, and they are all actually massive.

So they're the force that's pushing the field to the edge of the universe? Well, the electromagnetic and the weak interactions. Now, the gravity is actually a bit of a problem. There's another problem with gravity, and that is you can't actually describe it. it's a game with a boson, the graviton is a boson, but actually it's a spin-two object. Now, why is it a spin-2? He's got two units of spin spinning around.

And the reason why you have to have this is that... So, spin around in the next one. You're wiggling an index finger, around, around, around like that. So it's like that thing that goes around, when you go to a fun fair, and you get all that stuff, it goes, whirls around a stick. Fluff, whatever it's called. Yeah.

That's right. Candy floss. Indeed. And it's, I mean, and it comes with the things, if the candy flask comes in one unit or two unit or three units, and that's what we mean by the spin of these particles. Now, the graviton is a bit different because what we do observe is it all matter attracts.

And to make that possible in a field theory, quantum fuel, you have to have a spin-two particle. And that's a bit different from all the other ones that we've got at the moment, which leads to another problem in trying to put generality, and gravitation together with the other things that we think we've got a good description of. You leaned across to Jim then, so I'm going to lean across to Jim equally,

maybe not asking the question you were implying, asking by the gestures you made, but simply say, can we go back of it then, Jim McAulilly, and say, bring in the name Plank and somehow that might be useful in this discussion? Yeah, Max Planck, who's a German physicist, who basically was the founding father of quantum theory.

In 1900, he was the first person to propose that radiation, heat emitted from bodies can only really be described, or the description required for it to fit experimental data, was to imagine this heat in terms of tiny lumps of energy. Irreducible lumps, quantized. That's right. That energy wasn't continuous, but at the tiniest scale, it came in tiny discrete chunks, the quanta.

So usually when people talk about a quantum leap, meaning something very big. In fact, that's quite wrong. A quantum leap is the tiniest jump you could possibly make. It really annoys me when I... A lot of journalists are readjusting their microphone. Thank you. As you speak. It'll be glad of that.

It'll join decimated. So, but Plank was a very reluctant revolutionary. He really didn't like this idea. But nevertheless, he was the person who got started, this idea that energy, was irreducible at the smallest level. And people like Einstein came along later and said the whole of electromagnetic radiation does in fact come in tiny lumps,

which we now call photons. And so Plank's name is forever associated with the idea of chopping things up and quantizing them into the smallest, discrete elements. We now think that we should also be able to, if certainly quantum physicists believe this, that even space and space, time themselves should ultimately come in tiny, discrete chunks.

So the smallest units of a volume of space, the tiniest unit of time, smaller than which it would not make any sense. And this is something that's called the Planck scale. And we believe we have to describe nature down at the plank scale before we can unify the forces. And maybe partly lies the problem with uniting quantum mechanics and general relativity. And the general relativity, if we think back to the model we had for gravity of our curvature in that nice rubber sheet,

it's very smooth. It's always a smooth curvature. You know, we can put on lumps of mass, different places on the sheet, but it's always nice and continuous. There's a nice smooth space time. So general relativity doesn't have built into it that lumpiness that we need if we're going to describe things in terms of quantum mechanics.

How small are we talking? We talk about gravitons, photons. Can you just give us some idea of the smallness of things? I can do that. Certainly, in terms of gravitational waves, those gravitational effects propagating that we're trying to detect. So if you imagine, again, a rubber sheet,

those ripples on it are propagating. Do you carry this metaphor around all the time? It's equivalent of your knapsack on your package. I do actually. You're a whole-purpose rubber sheet? I actually have a rubber sheet. I actually have a rubber sheet. rubber sheet.

So imagine two points on that rubber sheet. As the ripples pass through, two points on the sheet are stretched and compressed. So what the fluctuations in gravity are doing are changing the separations of objects in space-time. But the separations caused are tiny, and I can quantify that in a bit. but if you imagine we're sitting here, gravitational waves are passing through us all the time so we're being squished and stretched and compressed all the time but we don't really worry about it too much because the effects so small we don't really feel it. I think they just felt a squish just passing through.

So if you imagine you've got two masses, two masses and in-space time separated by say a few kilometres, they are only going to move a tiny, tiny amount. If they're separated by a few kilometres, they're going to move something like, and 10 to the minus 18 of a metre, what does that mean? It's like trying to sense the change in distance from our sun

to the next nearest star to the accuracy of a human hair. So it's tiny, tiny, tiny effects that we're trying to detect. Roger. It's only imaginable, actually. I mean, I've looked at some of your figure you with me and it just beats everybody.

Well, it can't beat you, chaps. you wouldn't be working where you are, but unleft standing at the 17th billion. I think we have great admiration for these people who try to detect these gravity waves, and they use very, very clever ways of getting at them. But of course, it keeps on coming back to this fact that gravity is very, very weak. Whenever gravity goes by you, actually does essentially nothing at all. And that's the real problem we have, but it's so much weaker than all the other things. This is one of the great problems for trying to bring gravity in life.

line with every other, all the other three forces that we know about. It's so different in strength. And, I mean, there are lots of interesting ideas about how you can try and strengthen up gravity to the level that we might be able to see these gravitons produced in the Large Hadron Collider. And that takes us off into another bizarre direction, which is an intersection of extra dimensions in the universe, in the sense that gravity has got, we live in more than a three space, four space time dimensions, three space of one time, that there are other dimensions in the world and that gravity is in those dimensions as well. And so it gets diluted because it's being used up in those other dimension, it ends up by being in the world we live in much, much weaker.

So gravity might be a signal from parallel of other universes, Jim. Well, there are certainly theories that would suggest something like that, but maybe that's jumping the gun a bit. In terms of higher dimensions, ever since Einstein, Einstein spent the last three decades of his life trying to find a theory that unified the forces of nature. He failed. At the moment, the search is on for what's called the theory of quantum gravity

that unifies quantum mechanics and general relativity. And it seems that the front-runner candidate theory that would unify all the forces is something called super string theory. And it's a highly mathematical theory, but it's one which does certainly bring all the forces together under one roof. The problem that most people,

and I include most physicists, have with it, is that conceptually it's very difficult to understand because it suggests that our universe is one, of ten dimensions. Now it's hard enough trying to think of time as the fourth dimension, something Einstein told us a hundred years ago, but to imagine there are these other hidden dimensions of space that we have no evidence for is very, very difficult. And that's led on then to even more exotic notions that maybe our ten dimensional universe is embedded within a higher dimension. There's all these parallel universes bumping into each other. And it's very, very

hypothetical at the moment and people have a lot of fun coming up with new theories but no one really knows for sure. But in the reading I did from you the suggestion was that there are now at least five string theories and that sort of weakens the idea of there being a string theory that's strong enough to take us any further. This was certainly a problem about a decade ago. People were worried that if we had a grand unified theory that was the theory of everything then surely it should be unique. once they had a proliferation of these different versions of string theory, they thought they were in trouble. But people like the American physicist Ed Witten

came up with the idea. He said, well, look, give me just one more dimension, an 11th dimension. And I promise I won't ask for any more dimensions, but if you do, I will combine all those different versions of string theory into one theory. When you're talking about another dimension, does this mean other universes,

or what does it actually mean another dimension? No, it doesn't mean other universes. It means another direction. and which is something we can't comprehend. Our brains are three-dimensional. And so we're aware of the three dimensions in our normal space. So in fact, what you're saying is that our brains aren't built for realizing the possibilities that might be there. That's right.

Because our brains are embedded in the three dimensions of space. Well, on the other hand, we only use 5% of our brains. So maybe the other 90% is lying in rating. Who knows? Who knows? It might be. Mathematically, of course. We can.

Yeah, I think that's a decision. I mean, it's difficult to immediately picture these things because we're used to picturing things in and three dimensions, and then we can add the time on in a sort of a bit of a convenient way. But that doesn't mean to say you can't write down the models, theories, and see the consequences of these other dimensions being there. They may be very difficult to detect. I'm picking up on what Jim said, this string theory is there's also precursor to that called supersymmetry, which would have to be there for a number of reasons.

Going back to this whole business of the graviton being a bit different from the other particles, the only way you can bring it in is if you have this thing called supersymmetry, which says for every particle you have another particle, which is slightly different in the candy floss that's spinning around. And you can get from one to another and on, from the spin one particles to this graviton. And with supersymmetry, you can start to bring them together. And that's a chance.

Now, one of the clues in super symmetry is every particle has a partner. We've therefore got to see all the other partners. It's going to twice as tricky. But we've got a large number of them at the moment. We've got all the matter. Now we want the super matter. And that, of course, is one of the big goals of gain something like the Large Hadron Collider at CERN, is that if these theory make any sense, then we've got to start seeing those super partners.

If you once get one of these superpartners, you say very warm, feel warm, and you say, fine, now I've got something which can start to relate the world I know to the supersymmetric world, which then takes us on to the graviton. So there's another thing that it was got to turn up if we're going to make any progress towards a graviton at CERN, and then we've got to find supersymmetric particles. And what's nice is that you don't have to find all the particles. Only one super symmetric particle discovered will tell us that we're on the right tracks. and supersymmetry is the way the universe behaves, and indirectly, that tells us there should be a graviton as well. Can we come back then to detecting the graviton? Sheila Rowan, can you tell us about the ways in which physicists are trying to locate it?

You're involved in the research on this. How at the moment, how is it going, this attempt to confirm the existence of the graviton? Well, we're trying to detect gravitational waves, which, as I see, are kind of large numbers potentially of graviton, so we're not going to see individual particles in the same way that you can with light and detect, you know, use the photoelectric effect to see individual packets of energy. We're going to see, again, the radio equivalent, the large numbers of gravitons. And those experiments are progressing pretty well.

And over the last maybe five to ten years, there have been some large instruments built to try and detect gravitational waves. There's a number of those around the world. There's a UK-German detector, which is actually built in Ruta outside Hanover in Germany, and there are two big US sites called the LIGO project. Ours is called DO-600. And the 600, I should explain a little bit about how these detectors work.

I said we're trying to detect changes in the relative separation of test masses. test particles. And so we build instruments to try and do that, sense changes and separation of mirrors, actually. And what we do is we use a laser light

to try and sense the changes and separation of these particles. So the detectors are like giant Michaelson interferometers, Michael's Biltys interphorometer, for a different reason. But these are large L-shaped devices in which we take a laser.

shine light from the laser onto a half-silvered essentially piece of glass, a beam splitter, split the laser beam into two, send it out a long distance, in our case 600 metres, bounce off what really are our test particles, mirrors at the ends of the L in the interferometer. That light bounced comes back, and these are light waves coming back, and if you imagine two peaks, if the light comes back such that two peaks arrive together, together, they add up and we'd see a bright spot if we looked at where they combined. If on the other hand a peak and a trough in the light wave come back, we would see a dark spot. And whether or not the light comes back with a peak or a trough depends on how far it's travelled.

And so we can, by looking at the bright or dark spot when the light comes back and recombines, we can tell whether or not the mirrors at the ends of the arms have moved, which is what we're trying to detect, that the gravitation, wave would come in and move these test particles. Jim, do you think this type of research helps us to establish the whereabouts of the nature of the graviton? And we're talking about something so extraordinary small, as Sheila said, we're looking for the movement of a hair

between here and billions and billions and billions and billions of miles away. Yeah. Do you think this is going to... I think in a sense, I mean, Sheila's work, as she mentioned, is really searching for gravity waves, which is a large number of gravitons flowing through space. In the sense, gravity waves research is there to basically add another confirmation to Einstein's general theory of relativity.

It's not, I don't think, going to directly lead to proof of the existence or the non-existence of individual quantum particles called gravitons. I think that is going to, A, come from theoretical, mathematical research in trying to find this theory of quantum particles. gravity, but also most likely indirectly from work done on Earth and in the large Hadron Collider in producing particles like the supersymmetric particle. Yeah, I mean, if I follow up on what Jim has been saying, that is that when we discover quantum mechanics and the photon, first of all, we already had electromagnetic waves.

We were in the fortune position. We had light and we had Maxwell and people like this that told us about electromagnetic wave. We'd had radio waves. So we're already in that part of the business. And then the next step was via, in fact, Plank and Einstein coming up with the quanta of the electromagnetic waves, which are the photons. So here we're pretty blind. We haven't got a gravity wave yet.

So I think you have to look at this as a sort of, for me, as a combined effort of, first of all, getting the gravity waves there, then you'll have to ask whether it can be quantized, and you'll do that theoretically, the way Jim has described to see whether you can make a models and theories work or experimentally, you'll have to try and find one of these gravitons and that's going to be jolly hard. Yes, I mean, I think we're not likely.

I don't believe we're likely to find a graviton. Gravity waves, I would, I don't know if I bet my mortgage that one day will find gravity waves, but pretty close. I think gravity waves exist. I don't think Einstein's theory is in any doubt. No, I think we're very confident that we will. detect gravitational weaves. Right. But gravitons, I'm not sure at all.

You see, there are two camps in finding this Unified theory. The people who start from quantum mechanics, they believe gravitons should exist. So these are the guys who are working on super string theory and super symmetric ideas. They believe that if their ideas are right, the gravitons should be there. But there's another camp

that starts from general relativity and says general relativity is right and let's see if we can reach quantum mechanics from general relativity. They don't believe the Graviton exists. They think there is no such particle, or certainly their theories don't predict the existence of it, so it's by no means sure that there is even such a particle.

So if it doesn't exist, you say... I would say, Roger Penrose, for instance, who've been on some of your programs before, would, I think part should be in that camp that quantum mechanics may not apply to gravity. And that, I think, it's an open question. It really is very much an open question that we've got to get theoretical input on,

and we've got to get experimental input on too. I mean, detecting gravitational waves will help in as much as they're the kind of classic, again, using an analogy, there's the kind of classical manifestation of this propagating gravitational effects. So from that point of view in some way it helps. So then if you want to go into the quantum world, coming from the classical to the quantum world, would bring us along from the classical waves into the graviton.

So it's a question. It's therefore very important to find the gravitational waves. It is. And also for other reasons. I mean, the reason we're looking for gravitational waves is actually not just to confirm general relativity. It's ticked every box that people have tried to compete again so far, so we really do expect to see gravitational waves. But what they will do is let us actually see the universe in a different way.

Because at the moment, really, when we do astronomy and we point our telescopes upwards, we really look at electromagnetic radiation on the whole, which typically comes from the outside of stars. And the reason we're talking about stars and astrophysical events and all of this is again because gravity is so weak if we're going to see any gravitational waves, we need a huge amount of mass accelerating. So it's got to be some astrophysical source

with a huge amount of mass like a neutron star or a black hole, something that's really warping space-time. Now, those are objects that it's quite different. difficult to study. If we can see the gravitational waves being produced by those objects as they accelerate, we can perhaps learn things about those objects. We can't learn in any other way, which then feeds back actually into other areas. Can't we just assume it and get on with it? Why is it so important to find it? And if we find it, what is that, where is that going to take us, Roger? Well, I think we've all been sort of going around this at a level. The real issue is whether you can unify gravity with the other. interactions or general relativity with quantum mechanics. And that, I think, is a big question.

Now, the world may work that way, as we physicists would like to have a description. It may not. And unfortunately, we're in the position of we've got to do theoretical work to get it together. We've got to do the experimental work to get together and see if we can really, really put, bring these activities into place. I mean, we have lots of beautiful ideas, very elegant ideas. I mean, Mr Higgs and his Higgs boson was a very elegant idea. We still have yet to see it.

We're confident that it'll be there, but until you've actually put the dot on it, then across the T, then you're not certain you've got the right theory and the right description. I said in my introduction that if this is, the Gravitin is found, then more might be found out about the 95% of the universe we don't know about. What if it isn't found?

What if it isn't that? Well, yes, I mean, this is another piece of the jigsaw that we'd like to put in the nature of what's called dark matter. Basically, everything we see out in the universe is a tiny fraction of all the matter and the energy in the universe. And one possible candidate for this dark matter that's invisible that we know is there because of its gravitational effect, but we can't see it,

might be what Roger refers to as the partner, super symmetric partner of the graviton, something called the Gravitino. So it might be that we detect this even more exotic object called a Gravitino that would lead us indirectly to say that, well, the graviton must also exist, but we'll also explain all this missing mass of the universe at the same time. But the Gravitino has the same sorts of problems as the graviton, in the sense that its interactions are all very weak.

And so you can't sort of stick up a lump of metal and watch a Gravitino bounce off it. It just goes straight through the middle of it. And that's how difficult we're trying to get at the Gravitinos and the gravitons. and because of these weak interactions, the only way, as I said earlier, which we touched on, was to hope that they'll get strong interactions is that we have these extra dimensions in the universe that we actually inhabit,

but we've not been able to inspect them yet. If they are there, then maybe the interactions can be stronger on short-distance scales, the sort of things we will probe in the LHC, very, very short distances of a million-millionth of a metre. And then you can shake up those extra dimensions and perhaps produce a Graviton.

Well, thank you very much indeed, and thank you very much for listening to Sheila Rowan, Jim Al Kalili and Roger Cashmore. Next week, we'll be discussing Thomas Hobbes, the 17th century philosopher, a great radical philosopher, rather underrated. He used to sing himself to sleep for his health. Thank you 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. forward slash radio four.