In Our Time - Theories of Everything

Episode Date: March 25, 2004

Melvyn Bragg and guests discuss the 30 year search to solve all the biggest questions in physics. At the end of the last century, brave voices were predicting that all the big questions of physics were on the verge of being answered by a Theory of Everything. The disparity between the physics of the very small would finally be reconciled with the very large, and the four forces of nature would finally be united with a single set of equations. It was suggested that with such a theory we might solve the riddle of black holes, unlock the secrets of the Big Bang, probe other universes and even uncover the mystery of travelling through time. But Stephen Hawking, who once said that with a Theory of Everything “we would know the mind of God”, has changed his mind and now says that it may not be possible after all. So what are the prospects for a Theory of Everything? Why do we need one? How do we get one? And what would it mean if we did? With Brian Greene, Professor of Physics and Mathematics at Columbia University and author of The Fabric of the Cosmos; John Barrow, Professor of Mathematical Sciences at the University of Cambridge and author of The Constants of Nature; Dr Val Gibson, particle physicist from the Cavendish Laboratory and Fellow of Trinity College, Cambridge.

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or wherever you get your pods. 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, at the end of the last century, brave voices were predicting that all the big questions of physics were on the verge of being answered by a theory of everything.

The disparity between the physics of the very small would finally be reconciled with a very large, and the four forces of nature would finally be reaffed. united with a single set of equations. It was suggested that with such a theory we might solve the riddle of black holes, unlock the secrets of the Big Bang, probe other universes, and even uncover the mystery of travelling through time. Now it's 2004 and the clock is still ticking. Stephen Hawking once said that with a theory of everything, quote, we would know the mind of God, unquote, has changed his mind and now says that it may not be possible after all.

So what are the prospects for a theory of everything? Why do we need one? How do we get one? And what would it mean if we did? With me to discuss, this subject is Brian Green, Professor of Physics and Mathematics at Columbia University, and author of The Fabric of the Cosmos, Val Gibson, particle physicist from the Cavendish Laboratory and Fellow of Trinity College, Cambridge, and John Barrow, author of Constance of Nature, and Professor of Mathematical Sciences at the University of Cambridge. Brian Green, can you start by outlining for us the central problem for modern physics? Why is it the physics of gravity and the physics of the quantum really don't fit? Well, that is the essential problem, trying to put together the two main theories of physics that were developed during the 20th century, the physics of gravity, general relativity, and the physics of the micro-world quantum mechanics. And indeed, both work extremely well in their own domain, but attempts to put them together into a single theory that would describe the big and the small has been very, very difficult. And I think one way of understanding why it's so difficult is to recognize that the central core of each thing.

theory is so different from the core of the other. General relativity in a nutshell describes space and time as a smooth geometrical structure, sort of like a rubber sheet that can bend and warp, but it's nice and gentle, a nice gentle curving geometry. Quantum theory, though, at its core, has something called the uncertainty principle, which says that there's always a certain amount of fuzziness, quantum jitter associated with anything in the micro-world. So you have this jittery character of the microworld versus this gentle character of the macro world. So putting them together into a single theory requires meshing those two contradictory views of the universe. That's why it's been so hard. And when was it realized that it was a problem?

It's been recognized for about 70 years that there's an issue. Certainly when people first developed quantum mechanics that was in 1920s and 1930s, they were only seeking to apply it to molecules, atoms and subatomic particles, and they found fantastic success. But this theory of gravity that Einstein had developed by 1915 was out there, waiting to be joined together with quantum theory. Little by little people took the problem seriously, but the more they investigated, the more difficult it looked to actually combine them. Is this stopping things happening, the fact that these two theories, as it were, don't gel?

Not in terms of things that we can go out and measure and look at and observe. So it's a rather esoteric problem from the point of view of things that we can actually measure. But there are key questions that we have asked in one form or another for thousands of years that this incompatibility is preventing us from answering, such as, how did the universe begin? That's perhaps the deepest scientific question I think you can ask. And the origin of the universe we believe was a realm when there was an enormous amount of matter crushed to a very small size.

So you need a fear of gravity because you're still. so much stuff. You need a theory of the small because the universe itself was tiny, which means you require... When you say tiny, can you say what you mean? Because when you people talk about tiny, you sometimes really mean you can't see, I mean that tiny. The kind of length scales that we're talking about here are so-called the plank length, which is in conventional descriptions, about 10 to the minus 33 centimeters. That's a hundredth of a billionth of a billionth the size of an atomic nucleus. So even by atomic scales, we're talking about something that is extremely small. But that we believe was the condition from which our universe emerged,

so we need a theory that can talk about the small and the big at the same time to understand how it all got started. When James Clark Maxwell brought together the force of electricity and magnetism, was there a feeling that the science was on a roll that it was all going to come together? He'd done that, so why shouldn't the others fall into line? Absolutely. I mean, there's well-known quotes from the late 1800s where the feeling was that science was drawn to an end, that the basic ideas and theories have been put

into place and all that we needed to is measure a few numbers to greater accuracy, and that would basically wrap it all up. So there was definitely a feeling that we were not only on a roll, but the role was coming to an end. And then the same feeling at the end of the 20th century. I don't think people were that naive. Do you just have this

that they overcome with the centennial optimism? Not at all. I mean, certainly people have thrown around words like we're upon the theory of everything and things of that sort, but most sober physicists recognize that there are so many things we don't understand and so much further to go. Well, to sober physicist, John Marrow, why does it matter that the laws describing the

movements of the planets are different from the laws describing the behavior of the atoms? I know that Brian Green has introduced us to that. Could you give us more on that? Yes, as he's indicated, we're in a situation where we have four perfectly good sets of legislation, as it were, which tell us how electromagnetic things work and how radioactive things work and nuclear things work and gravitational things work. And if we were content just to have those perfectly good working descriptions of things, it would be like living in a country that had four legislations in place at the same time. If you visit a country, you suspect that that's

not the case, that there's one single set of laws. And so people had been persuaded for a long time, really for aesthetic, perhaps religious reasons at root, that there's just one set of laws in the universe and that these four laws like different pieces of a jigsaw puzzle can be joined together to make a larger overarching pattern. And you hope that the joining of the pieces together, the unification, as it's sometimes called, of different laws, will place new constraints on the shapes of the pieces

or their colours, as it were, and maybe require new. particles to exist which allow the different legislations to talk to one another in a new way. So this is how you hope that you might test this idea. The other thing I think we've come to appreciate is that what this search for a theory of everything is about is about searching for the laws of nature. If we go outside, we never see any laws of nature. No one's seen a law of nature.

This is a rather platonic set of things behind the stage scenery. We see the outcomes of those laws and they're very complicated, perhaps infinitely so, things like chaos and complexity living this world of the outcomes. Yet the remarkable thing about our universe

is that the outcomes don't have to carry all the beautiful symmetries and simplicitys of the laws. The laws are much simpler than the outcomes. And if we have a quest to find this theory of everything, it could be really mathematically relatively simple compared with all the outcomes.

So there's two worlds really. There's the world that the physicists are searching to find the laws, and then there are the outcomes of those laws. They want to predict those so they can test the theory. But things like the works of Shakespeare, you and I, we're outcomes of those laws, but this theory is not really going to help us very much in understanding you and I or the works of Shakespeare.

Can I just take a point to the point? you made in that reply, which I thought of when I was doing research for this program, but felt it would be too rather stupid to bring up. But the idea of the theory of everything, how far do you think that derives from a sort of drive we have, the monotheistic drive? Well, I think it does, ultimately, people may not consciously think that, but our whole concept of laws of nature has an origin in monotheistic faith, also in particular types of civil legislation. It's a rather small step from the idea of the world being ordered by the decrees of some deity

to the concept of laws of nature. And we know that in societies in ancient China that were technically far more advanced than Europe, the idea of laws of nature did not arise because there was no belief in a single omnipotent deity. and we know in Europe the development of science took place in a culture where there was strong religious belief and sometimes religious persecution. But I think this is, it's a natural idea to go in tune with religious beliefs that have existed for a very long time.

Is it gravity, in these unification theories, is the subject of gravity the most difficult to bring into it, as it were? It certainly is. Why is that? Well, gravity acts on everything, and because of that, you can't turn it off. It's not like magnetism where you can cancel out a magnetic charge of one pole by bringing along another one. So everything feels the effect of gravity. You can't have a situation where you're gravity free, as it were.

And also, as Brian has said, the theory of gravity that we have from Einstein is much more sophisticated. than any of our other theories in a very unusual way. Other theories of nature give you little rules and regulations. They tell you how things move around on a tabletop, which is space. But you don't have to say anything about the tabletop. It's just there. You put things on it, billiard balls, they move around and collide. But in general relativity, the tabletop, as Eddington first asked us to think of it as,

is like a rubber sheet. And you introduce the particles onto that space, onto that sheet. and it deforms it, it determines the shape of space. And so if you have this quantum mechanical uncertainty, you can't know where everything is and how it's moving. You don't know the nature of the space into which you're going to introduce them. So you have a very nasty, vicious circle. Val Gibson, I've been talking, or I've been listening,

to two theoreticians. You're an experimental physicist. From a practical point of view, what would the discovery of a theory of everything bring for you? To what questions would it provide the answers? Well, I think the first question that it will provide the answer to is knowing a little bit more about the universe we live in. They also hopefully give us some information

about how the fundamental particles of nature and their interactions coupled together. And also if you know, if it can predict what they do, then you know what they're capable of and how you can use them. It would be nice to know a little bit more about what space time is. also whether other universes exist and maybe a theory of everything could actually define absolutely what time is

so it will define for you the start of time and it may answer questions things like could we instantaneously travel from one point to another I'm interested when you're working in Switzerland in this at CERD or not does the theory that's coming does that directly feed into what you do Or does it take a bit of time to, or are you running in parallel?

What's going on there? It's a very long process. I mean, the current theories of everything have been around for 20, 30 years. What we do in Geneva at CERN is actually to test ideas, which are inspired by those theories. So although the trouble is the theories at the moment are very much in their infancy, and the theorists are working in a very symmetric, unified world. and what they need to be able to do is actually describe the real world.

And at some point, the world that they're working in and the real world diverge, and they need to be able to explain how it diverges. And this is something we call symmetry breaking. So we live in a world where gravity is completely different to the other forces of nature. And so we can only look for signposts that would sort of indicate how to, to extrapolate back to a theory of everything. And we need to somehow meet in the middle, and that's the difficult part. Brian Green, can I come back to the Big Bang?

Why, knowing about that would be the solution to everything. Can you just give us more about what you think it's like, like, where the research is regarding that particular moment and so on? Well, what we've been doing over actually the last half century or so has been using the laws of physics and are ever a december. deepening understanding of those laws, to, if you will, run the cosmic film, the film that shows the evolution of the entire universe. We've been trying to run it further and further back in time to get a clearer sense of what happened in the earliest moments. And we do know that as you

run the film further back in time, the universe gets smaller, hotter, and denser. And the further back you go, the more tumultuous, the more energetic the universe appears to come. Now, we can use the laws of physics, depending on who you ask, to turn the clock back. certainly to say a tenth of a second after the Big Bang. But if we want to go even further back to about 10 to the minus 43 seconds after the Big Bang, that's the real prize. That's where we think this unified theory

really begins to flex its muscles because at that moment the universe is so tiny, so energetic that gravity and quantum mechanics have to be put together. Perhaps an analogy helps, because it's also relevant to the question of L. It's very difficult to imagine. Yeah, well, the universe being something so,

I mean, incredibly smaller than the point of a needle. Yeah, it is tough. It's tough for all of us. And it is what our theory leads us to believe, but it's very hard to picture. But there's one key point that probably is worth emphasizing, which is the basic framework of how the laws describe the universe does appear to change when the universe gets hotter and denser. For example, if you had a glass of ice water, you'd have in there water, you'd have ice, two separate things. But then if you heat it up, you know if you make it hot enough, the ice and the water all melts, and then, in fact, it all turns into steam.

It all turns into one uniform entity. We think the same is true with the laws of physics. So today, as Val was saying, we have, say, gravity and the other forces of nature, the ones John was describing, are completely distinct in the world around us. But as the universe gets smaller and hotter, like the glass of ice water, the laws begin to meld together. They begin to merge together into one uniform framework. That's the framework that we're working out. And the challenge is to go from that uniform framework

to the non-uniform framework that exists today. In other words, we have to learn how to cool down the laws of physics, allow them to freeze out in different ways. And that's how we would make contact with the physics that we can now observe in accelerators around the world. How does string theory help you in this? Well, string theory is, we believe, the only theory that's really capable of describing the uniform hull,

the place where gravity and the other fundamental laws all meld together. It's the first approach that is able to put general relativity and quantum mechanics together into one consistent theoretical structure. In the fabric of the cosmos, your new book, you tell us about string. I've got a quotation here. I much rather you told us what you think is told our listeners what the string is. Any idea of what is the string? Yeah, well, the basic idea of string theory in a nutshell is very straightforward.

You know, we have been asking collectively for thousands of. of years? What are the fundamental, uncutable, indivisible entities that make up everything in the world around us? So, I mean, if you take a piece of wood, you cut it in a half, you keep on slicing into smaller pieces, where does it end? What's the smallest thing that you can't cut further? Now, of course, we all know you get to atoms sooner or later, but we also all know that they are not the end of the line. They can be cut. You have electrons that are around a nucleus, which has neutrons and protons. And we all learn in school that even those have smaller particles inside called quarks.

Now, conventional theory, conventional experiment stops there. Electrons quarks, a few other exotic species that Val can fill in if you'd like, are the dot particles no finer substructure. Strings come along and challenge that picture by suggesting, suggesting that within every particle, electrons, quarks, and any other particle you've ever heard of, is a little filament of vibrating energy. The filament looks string-like, and that's where the name string theory comes from. And the string can vibrate in different ways, just like the string in a violin,

vibrates in different patterns which our ears pick up as different musical tones. But the little strings and string theory, these tiny filaments of energy, when they vibrate in different ways, they look like different particles, according to the theory. So a string vibrating in one pattern, like a C, would be an electron. The string vibrating in a different pattern, a B, might be a cork. So everything arises from the distinct vibrational patterns of these fundamental entities, these strings. That's the idea. What does string theory mean for the relationship between energy and matter?

Well, I don't think it has any particularly new message to give us. I mean, the famous formula that everyone in the street knows that E equals MC squared. This plays a role in what Brian's has been saying about strings. That once upon a time, people questing for this theory of everything, imagine that it was going to tell you the masses of all the most elementary parts, particles of nature. The trouble with that was that as time went on, there seemed to be so many of them. You know, what you expect it would be a rather exclusive club of just two or three particles. Turned out to be a all-come-as-bizarre, a huge number of possible elementary particles.

And what the idea of the string does for you is that by strumming the string, as it were, all the energies of vibration that it can have, by using this energy mass equivalence, E equals MC squared, they're equivalent to different masses of particle. And so the single vibrations of one of these super strings allows you in principle to predict what we call the masses of elementary particles. It's not quite as simple as that, of course, that the strings have a little tension which changes as the energy and the temperature of the environment alters. And as we come down to the cool world which we live in today

where there can be planets and biology and people, the tension in the string becomes very, very high and the little loops of string become more and more like points to a higher and higher approximation. And so you hope to recover all the successful predictions and experimental tests that you've made in the past of the world under the assumption that elementary particles are points. But when you go back to this,

very, very high energy, exotic world of the so-called plank dimension, plank temperature, 10 to the 32 times hotter than in this studio, the tension becomes very low, and the strings become stringy and line-like, and they behave in quite a different way. And the challenge is to extract all those predictions from this stringy environment. Can I go across to Val now, Val Gibson, because your experience, experimenting on this. I mean, do you think, does string theory remain an untested theory? I mean, we can't, is there any way we can know they're there as distinct from speculating and assuming they're there and building on that? Well, we can only look for signposts. We're not going to be able to observe strings per se in the next 25 years.

But we can look for certain signpost along the way. And the types of things we can look for are, the idea of extra dimensions, which we can look for in experiments, and looking for the graviton. The graviton is a ripple in the space time, which hasn't yet been observed. It's a hypothetical particle. What would those give you?

What would the extra dimensions give you, if you found extra dimensions? Well, if you go back to this picture that we had of the universe being in a brain, a membrane, imagine, we've had the example of balls on a, on a table, but we can also consider a trampoline. Then if you've got two people on the trampoline, right,

and one jumps, then the ripple along the trampoline, the other person will jump as well. That would be sort of gravity in our own space time. But imagine if one person jumps and nothing happens. That would be an observation of gravity going into the extra dimensions, going into the air. And so we can look for things like that where we put a lot of energy into a collision.

and observe some missing energy coming out at the other end. And that would be an indication of an extra dimension or one signpost towards a theory of everything. I still don't get why that reads you back to string theory. I mean, I don't get it. I'm sure you three know inside out, obviously you do, but I don't. So why does that take us back to string theory and why does that make string theory the best bet in town?

Well, in a nutshell, it doesn't. We can actually use extra dimensions. we would predict that extra dimensions exist just to solve problems with our current theories. But it is one of the consequences of theory of everything with string theory that you would predict that extra dimensions exist. Now how you actually say the extra dimension

of you observe and those that are predicted of the same thing, that's a difficult question. Yeah, I think that a key point is that you can pull ideas, interesting ideas out of a hat and that's what theorists are really good at. But the nice thing about string theory is that it's a theory which demands that the universe have extra dimensions. The theory just does not work mathematically in a universe that has only three spatial dimensions.

The theory requires six and actually probably seven more dimensions that nobody's ever seen, but the mathematics absolutely requires. And since we believe the universe should put together general relativity and quantum mechanics in one consistent framework, string theory does that, we take this bizarre prediction that the world has more dimensions than we see very seriously. And indeed, one way to test for it would be to do an old version of the coins falling out of your pocket and getting lost between the cushions. We think that there may be energy in high-energy collisions carried away by gravitons that seeps into the cracks of the extra dimensions, and in that way is missing at the end of the experiment.

You start out with a certain amount of energy and you wind up with less than you began with. And the explanation, if the energy is missing in the right pattern, is that the energy has been carried away until those extra dimensions that string theory requires. So you start with something you can't possibly ever see string theory, and the evidence for it is something that doesn't sound as if it's anywhere being discovered. Well, this is the basis. No, no, I wouldn't want to leave you with that impression at all.

So, indeed, it may be that string theory, as with many other theories, is proven indirectly. It may be proven without literally looking and seeing a string, but that's very familiar in science. And this one particular experiment would be looking for the signature, a very clear signature that there are extra dimensions by virtue of their ability to absorb stuff from our dimensions

and make it missing. Let me just quickly jump in with one more thing. I would like to emphasize that there are some maverick ideas in string theory in the last few years associated with these extra dimensions that we're just describing, which do suggest that we might actually see strings at the Large Hadron Collider at CERN in the next decade or something.

That is not ruled out. So there are these ideas that strings might be larger than we have long since thought, still pretty darn small, but larger than that plank length we discussed earlier. And if that's the case, then it may be that these high-energy collisions may be able to excite the vibrations of these strings. And a whole slew of new particles, new vibrational patterns, may show up in the detector in the next decade. I'll come to you in a moment, John, but do you think that's possible about working at certain with this 27-kilometer? you'd better explain what it is. It's beyond me.

It's a, as you say, a 27-kilometer tunnel where we inject protons in the clockwise direction and protons in the anti-clockwise direction. And we just get these protons from a bottle of hydrogen. You strip off the electrons with electric fields and you go through a series of processes with electric fields and magnetic fields, you can accelerate them

and bend them with magnetic fields. and you do that until they're up to the speed of light, and then you inject them into the 27-kilometer tunnel 100,000 million at a time in a little bunch, all concentrated in less than a tenth of a millimeter. And so these will go around in the accelerator, not losing their energy, and then at some points along the circumference of the accelerator,

in fact four points at CERN, we can just use some magnets and we can collide them. And we can put all that energy, which is equivalent to something like a 200-ton train traveling at 200 kilometres per hour, into a collision. And if we put all that energy in, then we can create through E equals MC squared. We can create these massive particles and search for them in the experiments. John Barrow, this seems to go against what a lot of people will be, who listen to this program, will be saying, look, the whole point about science is you prove these things, you test these things. These are experiments we can see?

Now, we're not talking in that way at the moment, aren't we? or are we? Well, we might be. What you have to remember is that it's very hard, if not impossible, for experiments to prove things. They tend to rule things out. So you can rule theories out, or you can change their likelihood of being true because you don't see things that they rather probably predict.

The issue with string theory is that it requires there to be many more dimensions of space than the three we walk around in, but it doesn't tell you how big they are or anything like that. And what physicists have assumed is that we walk around in three dimensions, they're very large, and we know that if there were more than three big dimensions, that atoms couldn't exist and we couldn't exist. So any other dimensions better be rather small. And we don't know in this theory whether three dimensions big

is something that's programmed and inevitable in the theory of everything, that it couldn't be any other way. or whether it falls out sort of at random, and in some parts of the universe, there may be five big dimensions and no string theorists and other places they may all be big. So we just don't know. We have an enormous range of possibility

as to what their sizes should be. And so the poor experimentalists have not got a very, very definite target to shoot at and to say, well, if we don't see that, the theory is ruled out. But they're on something of an exploration of a very large, range of possibilities. But one thing that the LHC, I think, was built to try and find, which is a crucial ingredient in these theories,

is something we haven't mentioned so far. It's called super symmetry. So super strings have this super prefix because the theory has a certain symmetry. It's roughly like a symmetry between matter and radiation. But it predicts that the world should contain sort of twin copies, as it were, of every species of elementary particle that we're familiar with.

So for the electron, there's a supersymmetric electron called the selectron. For the neutrino, there's a supersymmetric version, the s neutrino. So there's this huge collection of elementary particles that none of which have been seen yet, but some of which ought to be seen by the LHC, and that was why lots of money was put into it, to find some of those particles. course. It's being upgraded the super collider, isn't it, in 2007.

By 2007, it will be upgraded. Is it going to do these things that John was talking about? Well, we'll be certainly looking for supersymmetry and extra dimensions, as well as a plethora of other physics. I mean, there's so many unresolved questions and unresolved problems that are not answered yet. But as we're saying before, if you take all this energy, you can create a lot of mass. and the problem is, is we don't know what the mass of the super symmetric particles are. There's no prediction of the mass. So it would be lovely if the string theorists came along to us and said, look for this super symmetric particle and such a such a mass,

then we could look for it. But there is no prediction. So we don't know, in fact, whether we will actually discover supersymmetry. So you might be making observations which are crucial at the moment, but not know it, because we've talked about the type of predictions that would come out of this theory about whether they're extra, dimensions, new types of particle and so forth.

But ultimately embedded within the mathematics of these theories ought to be predictions of things like why the charge on the electron has the numerical value that it does, why some of the constants of nature have the values that they do. And if tomorrow someone, some string theorists, were to do a calculation which could predict the value of the fine structure constant correct at 20 decimal places, nobody would worry about all these other problems.

This would be such a fabulous success that it would really vindicate the general idea of the theory. So there could all of a sudden come a rather specific prediction of the value of something which we already measure, but we have no reason why it has the value it does. Yeah, that's an absolutely key point. One of the main goals of string theory beyond this unified theory that we've discussed

putting gravity and quantum mechanics together is to resolve some open issues in the current, very successful but incomplete theories of the elementary particles of nature. And one of the main missing pieces in that theory is there are a number of numbers that that theory relies upon, numbers like John was just describing, the electric charge of the electron, the mass of the electron, the mass of the corks and so forth. They're about 19 or 20 numbers, depending on how you count them, that we put in, based upon doing measurements, we measure the numbers, We then put them into the theory, and then we use that structure to make further predictions. And the further predictions really are borne out by experiment. But the question is, why do those numbers have the values that they do?

A very interesting question. Now, again, this could be something where you say, well, who really cares? You know, if the electron was twice as heavy or half as heavy, who cares? The world would still be what it is. But we've known for a while now that if you fiddle with these numbers, by even a small percent on some of them, the universe, as we know, it disappears. the universe, for instance, it has a key feature being stars that rely on nuclear processes that themselves demand interrelationships between these numbers.

You start playing with these numbers, the interrelationships are spoiled, stars don't light up, and the universe is a very different place. I don't understand that. I mean, you start playing with these numbers in one way in terms of describing the universe, you mean? That's right. So if you, for instance, were to change the strength of the electromagnetic force, or if you were to change the strength of the gravitational force

or the strength of the nuclear forces, then the processes that make stars do what they ordinarily do would no longer happen. So the stars wouldn't be there. So the universe, as we know it, would not exist. In one of your, in past your book, I think you talk about deep laws or basic laws.

What do you mean by that? Well, typically when we talk about deep laws, we are talking about the laws that can describe the fundamental entities, whatever they are, particles or strings, and the ways in which these fundamental entities interact. So indeed, we are talking about the very laws that I was just mentioning, you know, the gravitational law, the electromagnetic law, and the nuclear force laws. And we want to understand not only the laws, but all of the parameters, all the numbers on which these laws depend.

And that's where string theory can go beyond conventional theories, because it has the chance, it hasn't done it, but it has the potential to give us an explanation, a calculation of the mass of the electron and the strength of the electromagnetic force. And as John is saying, if we were able, we haven't yet, but were we able to calculate on a piece of paper with a pen and paper the strength of the electromagnetic force, the so-called fine structure constant, that I think would be convincing to many, many physicists, even if we don't yet have any experimental vindication of the theory. John Barrow, what kind of thing, what time did you hope to see happen at CERN if string theory is correct? Well, I hope first that the so-called Higgs particle and some supersymmetric particles are found at CERN. As a cosmologist, I have another interest in those particles, as you and listeners, I'm sure, have heard on other occasions. One of the great mysteries of astronomy and cosmology

is what is the missing component of the matter in the universe, the so-called dark matter. that the vast majority of matter in our universe that reveals itself by gravity doesn't shine in the dark. It's suspected to be some type of elementary particle. And almost certainly one of these new types of particles, one of these supersymmetric particles. So there's a sort of a twin payoff from finding these particles at CERN

that you would confirm the idea of supersymmetry and so establish one of the foundation stones of superstring. theory, but you might also solve, provide the candidate for the dark matter problem. The other thing that I'm interested in from these theories is the constants. These theories predict the world has many more dimensions than the three that we walk around in. And so the true constants of nature, these fundamental numbers that define the fabric of the universe, don't actually just live in our three dimensions. they dictate the structure of the ten-dimensional universe.

And we see three-dimensional shadows of them in effect in our world. And it turns out that those shadows don't actually have to be constant. So our constants can drift very, very slowly without destroying the ten-dimensional constancy required for the overall theory. And so some of us have been involved for some years in projects to search for evidence for very, very tiny variations in the constants of nature. over cosmic time

by looking at distant quasars and examining the atomic structure that produces the patterns of light from them, comparing them with similar light in laboratory. And we get a check whether physics was the same 10 billion years ago as to what it is today, 10 billion years being when the light left the quasar.

So there are, again, these little sort of chinks in this theory where if we're lucky, there might be some new type of evidence emerging. Brian Green, Stephen Hawking, in his recent paper Godell and the End of Physics, says that he now thinks that the missing physics, the theory of everything, as it were, may forever remain a conundrum. What's your view of that? Are you still convinced that string theory will provide a theory of everything, as it were? Well, I haven't read the paper.

I've heard of it somewhat, but my general prejudice is that any attempt to argue that there can't be, you know, fundamental laws that don't rely on yet further fundamental laws using any idea of Girdle, I suspect, is unfounded. If one studies Girdle's ideas in any detail, one finds that, indeed, in an abstract mathematical framework, one can prove that there are going to be statements that you can't decide whether they're right or wrong. This is a very shocking idea that was developed in the last century, and it's an interesting thought. But when you look at the mathematical propositions that you can't decide whether they're right or wrong, they're usually highly contrived, generally have very little relevance for things that we, for instance, as

physicists, would be interested in. So my own feeling is completely unchanged since it's been, since I was a graduate student here at Oxford many years ago, that indeed there are fundamental laws that the universe, whenever we look deeper, we find the universe gets simpler and simpler when looked at the right way. And I think simplicity does come to an end. There is a rock-bottom set of laws, a rock-bottom set of fundamental entities. Whether strings or them, I don't know. But we certainly, I think, will find them. I hope it's in my lifetime. I'd love to know what the answer is. But I remain convinced that there is a deep law of the universe that we will discover. Do you share that optimism or that conviction or both? As an experimentalist, I guess I have to be a bit more

practical. And an experimentalist takes one step at a time. So I can put my hand on my heart and say in the next five to ten years, we will discover something new. Now, whether it's the signposts for theories of everything and string theory, I don't know. We'd have to take the next step when we get there. What do you believe in your heart, though? Is there a fundamental law that we're going to finally come to? That's going to be it? I would like to believe that is the case. But I don't, I think we would be unusually lucky if it was us who discovered it. John? Yes, I don't think you should worry about Girdle stopping us finding a theory of everything. What Girdle showed was that there are statements of any mathematical system that's rich enough to contain arithmetic that you can't

show to be true or false. And so some people, you know, sometimes imagine that somehow one of those so-called undecidable statements might be part of what you require to create the theory of everything. But that is by no means required. Physics uses a tiny part of mathematics. The mathematics is an infinite subject. The only subject is infinite. And there is no reason why the theory of everything should use any mathematics that's in the undecidable part of mathematics. So the mathematics required for a theory of everything may be entirely decidable parts of group theory and so on. I think Stephen Hawking was particularly intrigued by the fact that cosmology

and the whole quantum gravity theory of the universe is a type of self-reference problem, you know, that we're part of the universe that we're trying to describe. And Gerdel's proof of his theorem made use of such self-reference problems that you were able to take these famous paradoxes, you know, like all Cretans or liars, one of their poets has said so, and to transform them into statements about mathematics, which were equivalent to the statement that this statement cannot be proved.

So Gerdl never stops any mathematicians and mathematical physicists in practice from doing what they want to do. There hasn't been an example like that. So you think that Hawking may be mistaken, or his pessimism may be misplaced? I think he's been overly pessimistic. Yes. Brian Green, if this successful theory does emerge,

what are its implications on their grander scale? I forbore to jump in earlier when Val talked about instant the going to another place, which is what we'd all like to do. Are we talking about things like that? I don't know. It's very hard to predict where basic discoveries lead you.

I mean, if you were to ask the people developing quantum mechanics in the 20s and 30s, what's this stuff good for? There may have been some really insightful people who could have guessed where it would have led, but I don't think so. And today we have cell phones, we have personal computers, we have CD players, we have medical technology, all of which relies upon the physics of quantum mechanics. So when we're working on string theory, who knows, if it's right, first off,

10, 50, 100, 500 years from now, where will it lead in terms of changing our world? I don't know. What I can say is, aside from the technological implications, which may be centuries down the road, I think string theory will give us finally, if it's correct, the deepest understanding of space and time. So I think we'll get a sense of what space is, is it a real entity, what's it made of, time, where does it come from, is it a real entity, what the ingredients that make up time actually are? And certainly, when you understand something well, it can be the way. the first step to being able to control it. So will string theory allow us to manipulate space and time and I don't know, do all sorts of wild things that now we only think of a science fiction? It's hard

to predict, but it's certainly worth contemplating and we just keep going step by step and we'll see where it leads. Well, thank you all very much and thank you for listening. We hope you've enjoyed this Radio 4 podcast. You can find hundreds of other programs about history, science and philosophy at BBC.com.com.uk forward slash radio 4.