In Our Time - The Multiverse

Episode Date: February 21, 2008

Melvyn Bragg and guests will be leaving the studio, the planet and indeed, the universe to take a tour of the Multiverse. If you look up the word ‘universe’ in the Oxford English Dictionary you will find the following definition: “The whole of created or existing things regarded collectively; all things (including the earth, the heavens, and all the phenomena of space) considered as constituting a systematic whole.” That sounds fairly comprehensive as a description of everything, but for an increasing number of physicists and cosmologists the universe is not enough. They talk of a multiverse – literally many universes – to explain aspects of their theory, the character of the universe and the riddle of our existence within it. Indeed, compared to the scope and complexity of the multiverse, the whole of our known reality may be as a speck of sand upon a beach.The idea of a multiverse is still controversial, some argue that it isn’t even science, because it is based on an idea that we may never be able to prove or even see. But what might a multiverse be like, why are physicists and cosmologists increasingly interested in it and is it really scientific to discuss the existence of universes we may never know anything With Martin Rees, President of the Royal Society and Professor of Cosmology and Astrophysics at the University of Cambridge; Fay Dowker, Reader in Theoretical Physics at Imperial College; Bernard Carr, Professor of Mathematics and Astronomy at Queen Mary, University of London

This BBC podcast is supported by ads outside the UK. What makes people want to believe in aliens? I'm Tristan Redmond, one of the hosts of the Global Story podcast from the BBC. Donald Trump last week announced that he'd be releasing the US government's UFO files. So why the renewed interest in life out there? And what deeper spiritual meaning might people be searching for? Check out the global story. We are serious journalists on BBC.com.

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, if you look up the word universe in the Oxford English Dictionary, you'll find the following definition. The whole of created or existing things regarded collectively,

all things, including the earth, the heavens, and all the phenomena of space, considered as constituting a systematic whole. That sounds fairly comprehensive but for the description of everything but for an increasing number of physicists and cosmologists the universe is not enough they talk of a multiverse, literally

many universes to explain aspects of their theory, the character of the universe and the riddle of our existence within it. Indeed, compared to the scope and complexity of the multiverse, the whole of our known reality may be as a speck of sand upon a beach. But what might a multiverse be like? Why are physicists and cosmologists

increasingly interested in it? and is it really scientific to discuss the existence of universes we may never know anything about? With me to discuss the multiverse of Faye Dauke, reader in theoretical physics at Imperial College. Lord Rees, Martin Rees, president of the Royal Society and professor of cosmology and astrophysics at the University of Cambridge, and Bernard Carr, Professor of Mathematics and Astronomy

at Queen Mary University of London. Martin Rees, before we get to grips with the other universities, those that we can't see, what do we know about the universe we have? Well, cosmology has been a history of expanding horizons. In the old days, we believe there was our solar system and the vault of heaven were the fixed stars painted on it, but for the last 200 years we've been aware

that the stars are like our sun and there's an entire galaxy of stars. Since the 1920s, we've been aware that our galaxy, containing about 100 billion stars, is just one of many billions of galaxies which are spread through the part of the universe we can see. And in fact in the last 10 or 20 years, we've come up with a fairly standard picture of how the universe we can observe started off in a so-called Big Bang, a mysterious hot-dent state, about 13.5 billion years ago, and it's been expanding, and we can see these galaxies out to a distance of 10 billion light years or more, and as we look far away, we look back into the past. but there's still a sort of horizon

which we can observe because there's a limit to how far light's being able to come in the time since the Big Bang. So the part of the universe that we can see with our telescopes, even in principle, may not be all there is. It's rather like if you're in the middle of an ocean you look around you and there's a horizon

but there may be something beyond that and the issue which is challenging us now is to ask how much there might be beyond the part of the universe that we can actually see with our telescopes. Could there be galaxies that are unobservably far away, which are the aftermath of our Big Bang, could there even be other quite different Big Bang?

So that's the kind of challenge, which is enlarging our horizons one step further. So we've got larger horizons from Copernicus to seeing other galaxies, and now perhaps realizing that what we can see with our telescopes may be a tiny part of physical reality. Are there any limits, theoretically, to what we can, we'll be able to see?

Will there be more and more and more powerful telescopes? We will be able just as a matter of practical viewing to see more? Well, certainly we'll be able to look further back and understand better the distant parts of the universe, but there's this limit in principle set by the fact that light can't travel more than a certain distance since the Big Bang, so there may still be parts of space and time that we can never directly observe. And is that a bar to actually any conjectures or theories about multi-university? Well, it means that we can't directly observe these potential parts of the universe and certainly we can't observe other big bangs.

But I think one important philosophical question is to what extent it is part of science to talk about these regions. Because after all, we believe in Einstein's theory of relativity, but we can't observe inside black holes, although we believe what I'm, Einstein's theory tells us about it. So in science, we have to have some reason for believing in the theory, but we don't need to be able to test all its consequences. And what we want to know really is, can we infer anything about what might lie beyond the part of the universe we can actually observe?

So you use the word belief a lot throughout, Martin. We believe in... Well, I think it's very important that we should be open-minded, because as science advances, the area of consensus, the area of consensus grows, but new questions. come into focus, which couldn't even be imposed before. And the idea of our universe evolving from a hot-dend state was complete speculation 50 years ago, whereas now I would say that that's part of serious science,

which you should believe as much as you believe what a geophysicist tells you about the early history of the earth. But, of course, having made that progress, we now confront a new set of questions about what might lie beyond the part we can see, what might have happened right at the very beginning, etc. Fadaka, how scientifically useful do you think it is to step over that boundary, both in time and space, and try to establish what might be beyond it or before it?

Well, my views on this have actually changed. I've changed my mind. I used to think that it was not part of science to speculate about regions, other patches, beyond our own observable universe that we could never observe. But I think that, as Martin said, if you could have a theory which was well-tested and well-founded, that you had confidence in, that predicted that these other patches, unobservable to us, yes, but these other patches were there, then there would be good grounds to believe that they were there,

that they really did exist. But of course, because these consequences, these other universes are in principle unobservable by us, we'd have to be very, very sure about this theory that predicted that they were there. So the more distant observable phenomena or entities are from our direct observation, the more confident we have to be in the theories that describe them. So, for example, we don't have to have much of a scientific theory to believe that this table here exists,

but atoms and molecules are a different matter. It took hard scientific work on kinetic theory of gases and on Brownian motion. It took quantitative predictions about observable phenomena that were actually checked and verified in order to establish the reality of atoms and molecules. So this is the ultimate distance that we can get from observability if these other patches, other branch universes exist that we can never directly access, then the confidence that we have to have in the theory

that would predict them has to be so much greater. Bernard Carr, one idea about the early universe is called inflationary theory. Can you explain how this might lead to a multiverse possibility? Yes, but maybe I should first put it in a broader, the context. Most of the ideas of a multiverse come out of the attempt to try and explain how the universe was created. I mean, almost all cosmologists now believe the

Big Bang picture, which Martin has described. But the question is, can we explain the Big Bang itself, the actual creation of the universe? And until about 20 years ago, the answer would have been no. We can't explain the Big Bang because all physics breaks down at that infinite density. But the remarkable thing in the last 10, 20 years is that cosmologists have come out with all sorts of theories as to what might actually happen at the Big Bang itself. In other words, they have theories explaining how you create the universe. Now, clearly, if you have a picture for creating one universe, you have a mechanism in principle for creating lots of universes.

How are they testing these theories, Bernard? Well, the question of how you test them is a very tricky one, because you've got to bear in mind that actually there's not just one theory. for creating universes there's probably half a dozen theories which people work on. So you have to take each of these theories on its own terms and say, how can you test it?

Now, you asked about the inflationary universe, which is one particular model for the, if you like, the creation of the universe and the question of how that can be tested is very interesting. The standard Big Bang picture says that the whole of space and the galaxies are expanding, but expanding relatively gently

as a nice power of time. But according to our theories of what happened at very early times, the universe may have gone through a phase where it was expanding extra fast. In fact, instead of expanding as a simple power of time, it was expanding, as we say, exponentially with time. It was actually accelerating. And the reason for this is to do with the nature of the vacuum and things like that. But at least it's a popular idea.

Now, if this idea is correct, it means that our universe, our observable universe, as Martin described it, is actually just a very small part of a much larger bubble, which will extend way beyond what we can observe. I don't get that. Why has it got to be that? Well, because the nature of, maybe I should make this a little bit more quantitative, if we take our universe now, our observable universe now, if we go back to the time at which inflation occurred,

which was very, very early, something like a billion, billion, billionth of a second, that whole universe would have had something like the size of a grapefruit. And the idea is that that grapefruit itself resulted from something which was microscopic, which went through this inflationary phase and grew to a grapefruit. But the point is that there must be a region way beyond that grapefruit, and that region way beyond the grapefruit is what is beyond our presence of observable universe. But the crucial point is not just that there has to be a universe beyond what we're, can observe. The same theory

also predicts that there must be other patches, other bubbles, if you like, where the laws of physics are different. So inflation itself predicts not just that we are a minuscule part of a much larger domain, but that actually there are lots of other domains as well. And

in each of those domains, the laws of physics may be different. And those are what we call the multiverse. So, I mean, there's really two distinct steps. There's the step Martin referred to, which says that our observable universe is just a small patch in something much larger, but then there is the statement that actually

there are many other patches. And it's that second point, which is really so crucial to the idea of multiverse. Martin. I think it's important to realise that we can observe what the laws of physics are out to the limits of our telescopes, and it's

in a sense surprising that they are the same everywhere. We can analyse a light from a distant galaxy, and that light is emitted by exactly the same atoms as the kind of as we can see in the lab. So the laws of physics seem to be the same throughout this entire huge volume

that astronomers can observe. But of course, as we've discussed, that may be a tiny fraction of what's out there. And so it does become a genuine question. Could there be other domains far, far further away, or the aftermath of different Big Bangs where the laws could be different? Faye Daka, science is used to,

dealing, as Martin Reis pointed out, in his remarks, with the unobservable, black holes. theories are drawn from black holes, quarks, which nobody has ever seen, never will see, and they've built this massive collider to try to get closer. So the idea of working with the unobservable is not foreign to modern science. That's absolutely right, although there is perhaps a difference in principle in the sort of unobservability that we're talking about here.

So, for example, it's true that no one has ever seen a quark, but the theory that describes the quarks has causal implications. The quarks are the particles that we believe make up protons and neutrons, which we believe constitute the nuclei of atoms. So the behaviour of the quarks and the way that they interact would cause events that we could then directly test. Whereas the unobservability of these branch universes, these other patches, anything that happened in those universes

couldn't be a cause of some effect that we could see. So there's some perhaps difference in principle. in this kind of unobservability? There's a question which always comes up when you talk to philosophers, and that is that they say, as you said in the introduction, that the universe is, by definition, everything that exists. And, of course, if we stick to that universe, then the word multiverse is an unfortunate word.

But I think for the moment, so long as the idea of other universities is conjectural, it's best to stick to the world universe for what we do observe and then talk about multiverse. but if the multiverse concept eventually became firmly established, then we might need to redefine what we now call the multiverse as the universe, but then we need some other word, like say, meta-galaxy for the part of the multiverse that we can directly observe.

Fadaka, as I understand that you work on the origins of the universe, most people listening to this program, not all of them will have heard about what's called the Big Bang, but can you explain something called the Big Bounce and why that might lead to a multiverse? Well, there are many different sorts of multiverse theories or proposals for what a multiverse might be like. And one of them is that the observable part of our universe

is just one, the latest epoch, in a whole cycle of cosmic expansion. So there would be many big bangs followed by a period of expansion of the universe, followed by a recalapse to what would be called a big crunch. Subsequently, there will be another big bang and another expansion and another crunch event. So these epochs would follow one after the other.

And we can conjecture that the laws of physics could be different in each of these epochs and therefore it would be a type of multiverse. The point is that the picture I'm talking about of inflation essentially has a huge number of universes which are sort of spread out in space. but the picture that Fay talked about, the bouncing universe, is you've really got a large number of universes

which are spread out in time. But in both cases, the idea is that you've got many different universes. And that's only two examples, but there are other examples of well that one could talk about. Just to mention one other, there's an idea that involves extraspacial dimensions. You can imagine a whole lot of bugs crawling around on a big sheet of paper,

thinking that's a two-dimensional universe and being unaware of another set of bugs calling around on another parallel sheet of paper, which is a separate two-dimensional universe, as it were. And likewise, you could conceive, and there are some theories that would have this implication, that there could be another universe just a millimeter away from ours,

but we're unaware of it because that millimeter is measured in a fourth spatial dimension and we're imprisoned in R3. So that's just another idea. I should emphasize all these ideas, are speculative at the moment because they're based on what the physics was like at the extreme early stages of the universe

when conditions were very, very dense, very, very hot, far beyond the range we can directly test here on Earth. So that's why all these ideas are as yet very speculative. But the hope is that one day we will have a theory which we can test in the lab, which does give us indications as to what the physics was like at this extreme early time, and that can tell us whether the physics,

the idea of inflationary university with Bernard Carr mentioned is the correct one or not. These are speculative ideas, but there is hope that we will be able to narrow down the options among them in the next decades. Can you tell us the place that the Anthropic principle plays in all this? Well, there are many different ideas, statements, ways of arguing which people call anthropic. And some of them seem to me to be basic common sense. and are hardly controversial at all. Some of them seem to me to be quite wild and I don't set much store by them.

Let's start with the least controversial type of anthropic argument. And that's just the statement that when we're doing our science, we ought to take account of the fact that we're not just random observers in the universe. We are special. We are carbon-based, complex life forms, living on a rocky planet orbiting a average main sequence star late on in the evolution of the universe. And we see the universe now at such a late stage in its evolution because of the sort of beings we are. We're made of carbon mostly.

Carbon is produced during the life cycle of stars. So stars have to have been able to form and die in order for us to be here. and that means that we must exist when the universe is old and that will have just that realization actually can give us some explanatory power, some scientific power even without considering the possibility of a multiverse we can use that to explain the question

of why the laws of nature are the way we see them. Would you address this question, Vernon Carr as well please? I read in the notes for the program that the anthropic principle is based on fine tuning of certain aspects of our universe towards the existence of life. Can you just develop that? Yes. The point is that there are various forces in nature and these each have a particular strength. They're called the coupling constants. And we don't know, we cannot predict what the strengths of those coupling constants are.

At least we come with present physics. But what is found is that there are relationships between those constants. which seem to be required in order that we can be here. In order that galaxies can form and stars can form and chemistry conform and ultimately therefore human beings and intelligent beings can form and ask these sorts of questions, you have to have these fine tunings between these constants of nature. And it's not just these coupling constants,

it's the masses of the elementary particles, it's the cosmological constant which describes the acceleration of the universe. Now, known physics does not explain these fine tunings. It seems indisputable that these relationships are required in order that life can arise, and they're really quite precise. They don't determine constants uniquely, but they do determine constant satyr within something like 10%. And there simply is no explanation. And actually myself and Martin, we wrote a paper some 30 years ago, in fact,

where we put all these coincidences together, pointing out that these fine tunings were required. Now, at the time we made that suggestion, it was not a very popular idea because it was regarded as a somewhat metaphysical explanation because there was no idea as to how these fine-tunings would come about. I think there was maybe the suspicion that it hinted that there was some sort of fine-tuner or God, if you like, you must have created the universe in order to make life arise. And that was very unpopular among most physicists, because most physicists do not want to bring in a creator.

So what's been exciting about the multiverse is that if you believe there is a multiverse, there's all these universes where the constants are different, then it is fairly natural to say that there will be a small fraction of these universes in which the constants have the values which are required for life to arise. And so nowadays, I think many cosmologists regard the multiverse as the sort of legitimisation of the anthropic principle, because if there's only one universe, it's really rather hard to explain. But if you've got many, many universes, it is simply a natural selection effect that we have to be in a universe where the constants have the values which

are required. Martin, sorry, very first and then, Martin. So the existence of the multiverse, if we can establish it, would eliminate the question of why the laws of nature are the way we see them. Why they are this? Because in the multiverse, there is no this, they are everything, many different possibilities. So we have to rephrase the question. The question then becomes, well, if there are all these many different possibilities, why do we see these particular values? And then in the context of a really existing multiverse, that can then have the answer that, well, we see these particular constants or laws of physics in this particular range, because that's the only kind of universe that we could have evolved in.

And if we are, Bonica, that brings us back to an idea that people like to have enjoyed thinking about and believing in four centuries, which is that this place is special and unique. Well, if by this place you mean our universe, our observable unimuth. Well, I don't talk in smaller terms than that. I mean, that of course is precisely the concept which the multiverse is going against. But then in some sense, that's what the history of science has done. been. We've always wanted to think we were unique. We wanted to think the earth

was the center of the universe. We know that's wrong. Then we thought the galaxy was the center of the universe, now we know that's wrong. Now some of us like to think that the universe is all there is. But the observable universe is all there is. But now I think we have the glimmering of evidence that that is wrong. It's just another step in this progression of if you like of continual humiliation, if you like. And I think the important point to me... But is humiliation when so far there's no

discovery of and none of you have said in the conjectures of something producing what is here well it's humiliation in the sense that homo sapiens seems to be very very

insignificant in terms of scale compared to the size of the universe that's not insignificant knowledge isn't insignificant no well exactly to my perspective our understanding our attempts to understand the nature of the universe, that is the triumph. And to me, that humiliation is only at first sight. I mean, in terms of physical, our physical significance,

mankind is, if you like, insignificant, because we've only existed a short time. We could be wiped out, you know, by an asteroid or something like that. But the point is you think in bigger terms, and what is remarkable to me is that intelligence of some form has arrived in the universe. have to be homo sapiens, but the universe is designed for intelligence, and the remarkable thing is that in just really a matter of centuries, we have managed with our brains to develop an understanding of the whole of the existence, from the very large to the very small. And to me,

that is, if you like, the compensation for the humiliation involved in our physical insignificance. But to me, it is our ability to intellectually grapple with the nature of the universe. To me, that is, you know, that is the challenge. And that's the excitement. Martin, my very. It is rather wonderful that we can trace the chain of emerging complexity, whereby from this simple beginning in this hot, dense state, the first atoms, the first stars and the first planets formed,

and then the laws allowed on some of those planets, a complex biosphere to evolve. We can understand how this happened. We can observe how it is happening in other places. And we can also study how this is a consequence. of the laws of nature having particular values which end up with a universe that can host this complexity rather than being sterile and stillborn, as it were.

So we are in a special place in our universe, we are not in a random point of intergalactic space, we're on a special planet around a special star, and of course we're not in a combination, there's further complexity can emerge. But I think just as we are in a planet, which is especially important, and specially developed,

so we can now regard our entire cosmic volume that we can observe as being perhaps rather special on this still rander scale, because it's one of those volumes that has had a set of laws governing, which does allow this complexity. Most of the other bubbles in Bernard Carr's terminology would end up sterile or stillborn,

because they wouldn't allow this complexity. On the other hand, of course, there could be a few which are even more complex than ours, and our brains couldn't envisage that complexity. So we shouldn't say that we are the most complex possible organisms in the most complex possible universe, but we can say that we are in a rather special cosmic context, not only on a scale of a planetary system,

but also on the scale of the universe that astronomers can observe. One point I would like to make, following up what Martin said, was that it's important to stress that all these ideas of the multiverse, they haven't been constructed simply to explain these anthropic fine tunings. Physicists, both cosmologists and particle physicists,

they have independently come up with models which predict there are these other universes. And since the multiverse, therefore, is a sensible thing to discuss, that is a natural explanation for the anthropic fine tunings. And that is why, in some sense, the anthropic principle has now become more respectable. And although the word anthropic suggests, you know, means man, it's the Greek word for man, it's really nothing to do with mankind, that would be far too

conceited, it's just the condition that life or intelligence of some form should arise somewhere. I think it's important to realise that we can observe what the laws of physics are out to the limit of our telescopes, and it's, in a sense, surprising that they are the same everywhere. We can analyse a light from a distant galaxy, and that light is emitted by exactly the same. atoms as the kind of atoms we can see in the lab. So the laws of physics seem to be the same throughout this entire huge volume that astronomers can observe. But of course, as we've discussed, that may be a tiny fraction of what's out there. And so it does become a genuine question.

Could there be other domains far, far further away, or the aftermath of different Big Bangs, where the laws could be different? Can you explain to us how the question of the multiverse and of the universality of the law of physics relates to the search for a fundamental theory or the theory of everything? Well, we are searching for a theory that unifies the very large and the very small and we don't know the nature of that theory.

And in particular we don't know whether that theory when we have it will give us unique, formally, as it were, for the basic numbers in nature, the strength of gravity, the mass of the electron, etc. It may do. If it does, then those numbers

are just a brute fact and there's no role for anthropic selection arguments. But it's thought by many people that when we have this fundamental theory, it will not predict uniquely things like the mass of the election and the strength of gravity. They will be, in a sense, almost like environmental accidents from the way our big bang cooled down and different big banks would have cooled down differently. And so the big challenge for theories is to answer the question what aspects of physics are truly universal and what are in some sense environmental accidents,

though on a scale as large as our observable universe. To give you an analogy, think about snowflakes. Snowflakes, as you know, have variety of different shapes, but they all have hexagonal symmetry. The shape of a snowflake in detail depends on its environmental history, the detailed humidity of the cloud in which it formed, etc. But the fact they're all hexagonal is due to the fact that at the bedrock level, that's a feature of the water molecule.

So in the snowflake, there's the hexagonal nature, which is fairly fundamental, but the other things are environmental accidents, the detail pattern. Now, what we'd like to know is which of what we now call the laws of nature are really fundamental, and which are, environmental accidents, local bylaws in our cosmic patch, as it were, which arose because of the way our particular Big Bang cool down would be different elsewhere. And until we answer that question, we won't know what the actual variety is of conceivable universes, and we won't be able to actually formalize these anthropic reasonings.

But that's the key question, I would say, for theories like string theory, which, of course, is being followed in detail, but may or may not pan out. take us on with the string theory is what multibus might a string theorist envisage, I? A large sub-community of string theorists now believe that string theory predicts not one unique vacuum state. That is one set of laws for particle physics, how they interact with each other, how many particles there are. But a large number of those, and people have speculated or tried to do calculations about how many vacua there are, and the number 10 to the 500 seems to be bandied around a lot. If that's the case, and there's still not a consensus on that, even amongst string theorists,

then one might have the picture then that coupled to some model of the early universe, such as inflation, that each of these different vacuor could be realized in a universe and the multiverse would then be formed out of bubbles or patches, in each of which one of these vacuaries the description of reality. So that's the sort of ideas that some string theorists are playing with at the moment. You said you were a convert to this.

What made you a convert? The idea there might be a multiverse. Earlier on in the programme. Yes. A combination of things. So in actually reading some of Martin's writings on the subject, I have to say, but also in my own research,

I came to realise that a particular model which arises in the proposal for quantum gravity that I work on could be thought of as a multiverse. So I hadn't thought of it in those terms before, and it is indeed one of these bouncing cyclic universe models where the different universes of the multiverse follow one after the other. And there is such a model, a toy model, it has very unphysical aspects, some of its aspects are very unphysical.

But there is such a model in our proposal for quantum gravity, and it has some very nice features which could explain at least one of the fine tunings for life that we require. And the model is a balancing universe in which each cycle lasts longer and is slightly bigger than the one before. So one of the fine tunings that we need to do in order to have the universe that we see is that the shape of the universe at the earliest times

had to be very, very flat. In principle, the shape of the universe at the early. early times, the shape of space could be very curved up on itself. And in a sense, that's what you expect. But in order to get the universe to be, as we observe it today, that can't be right. The space has to be very flat. And if you wait long enough, so late on in this cycle of cosmic expansion recalapse, you will inevitably get universes that are flat enough at the beginning to be the type of universe that we see.

you conjecture Martin how soon anything is going to come of this that ceases to be belief and enters into the library of knowledge? Well I think we can narrow down the range of options by straightforward cosmological observations

in the next decade. But I think the fundamental question of unified theory and testing which of the laws of nature are truly universal is going to be a longer haul, as it were, because one thing which I think is common to all the theories that have been discussed is that

we have to understand the nature of space itself in a fundamental way and the scale of structure in space itself is a trillion trillion times smaller than atoms, so very, very far from direct observation. And I think it's clear that many of these questions can't be answered until we do understand the grainyness of space or the structure of space on that tiny, tiny scale on which it's very complicated indeed. Now, I don't know how long that will take. Indeed, some people might believe it may be beyond human brains

to actually achieve such an understanding. We shouldn't without that possibility, but I think it'll be a long haul before we are sure about any of these things. But I regard it as part of science, albeit speculative science, rather than just metaphysics. A lot of physicists have changed their minds on this issue. We've been looking up the statistics, something like, let us say, 20 years ago, 5%

and now far more than that. I think that there might be multiverses and what is, among you people are working at the highlight, what's a favoured theory? What do you think, and how are you going to test it? I keep wondering how you're going to test it. The

favoured theory in terms of just sheer numbers of people working on it is string theory. More theoretical physicists work on string theory as a proposal for a theory of quantum gravity than any other proposal. The approach that I take to the problem of quantum gravity is definitely a minority approach

compared to string theory. There's probably a dozen of us in the world working on the proposal which is based on the idea that space and time are granular and atomic at the smallest scales. And the way that these theories are going to be tested just depends on the theory. So, for example, this granular theory of space time could be tested by seeing how

the granularity could change the propagation of light, could affect the propagation of light from very distant sources. We might be able to detect the fact that space time is not smooth but is in fact made of these grains by seeing its effect on the light that reaches us from astrophysical objects. The tests are therefore very specific to the particular proposals. That that particular test would not arise in a different proposal of quantum gravity. Could I give one example of, for example,

how you might test inflationary theory. No, no. Because one of the remarkable predictions of inflation theory is that there should be little tiny density fluctuations in the universe of quantum origin, and this is predicted. And it's those fluctuations which we think eventually will give rise to galaxies and stars, in other words, to our own existence. And it had always been a puzzle.

Where do those fluctuations come from? Now, one of the most exciting developments in the last 10 years has been the fact that we can now see those. fluctuations in what's called the background radiation, which is the radiation which bays the whole universe and is the remnant from the hot Big Bang. And those little fluctuations in the temperature, which are very tiny, there's something like one in a hundred thousand, can be analyzed. And the form of those fluctuations has the form which is actually predicted by these inflationary theories. And really that is quite remarkable because you have theorists who are for 20 years of

been trying to work out what these predictions are inside their heads using their mathematics and their equations. And then just five years ago, they see these patterns in the sky on the background radiation, and they conform exactly with what was predicted. So it is, in that sense, it is testable. Is that the theory that you favour? The possibility that you favour? It's among cosmologists, it is probably the most popular one. And the reason being that the theories of physics do, in fact, principle predict that this could happen. We don't fully understand, obviously, what went happened at such a very early time, but we know that in principle the laws of physics could allow it. And that is a wonderful idea, because it relates the huge structures we see

when we look out into space, galaxies and clusters of galaxies, to tiny quantum microscopic fluctuations at this very, very early stage in the expansion of our universe. So it's a wonderful link between the very large and the very small. And I think we all realize that if we are to make progress in understanding the early universe, we are going to need a better theory which does link together the very large, the domain of gravity and Einstein's theory,

with the very small, the domain of quantum theory. Because back at the very beginning, the whole universe was so small that quantum effects are important for it. Whereas now we think about quantum effects when we talk about individual atoms, but when we talk about stars and galaxies, we don't worry about them,

but the link between those two theories is essential if you want to actually make progress in understanding the very beginning. It's because for the very reason that I work on trying to find a theory that would link together gravity and quantum theory, to find a theory of quantum gravity that I'm particularly interested in these cosmological questions.

So, as Martin said, the regimes at which gravity and quantum mechanics become important are very, very extreme. It's going to be very difficult to find any kind of direct. lab test of any of these theories. And therefore we look out to cosmology, we look back to the consequences of the Big Bang in order to test our theory.

So there's an interplay here between the theories that we're working on that will be theories of quantum gravity and cosmology. So the theories that we propose as theories of quantum gravity will hopefully inform cosmological models, we'll be able to make predictions about cosmology. but then the observations that we make can then constrain the forms of the theories that we propose. It really is crucial whether these theories will be actually ever testable because people say that the multiverse is not part of legitimate science

because you can never see the universes. But the point is the multiverse is predicted by these theories, such as String Theory as Faye has described. And so the question is, therefore, if we can test the theory, the string theory or quantum gravity theory or whatever, that will be indirect evidence, if you like, of the multiverse. But the problem is, of course, can you actually test the string theory?

And there's a big controversy among physicists at the moment as to whether these theories really are physics or whether they are more mathematics, whether you can actually test them. Now, the hope has always been, of course, that we will be able to test the string theory or M theory to use the most popular version of it now. in the laboratory, but it is conceivable that because the energy is involved in M theory is so much higher than anything we can test in the laboratory, that it might never be testable.

Now, if that were the case, if it were the case that you could never actually test these theories, then the question would be, is M theory itself part of science? Is it really just mathematics rather than physics? and of course we all hope very much that there will eventually be tests of M theory or quantum gravity, whatever it is because that is what is really in my mind going to make this all part of legitimate science. There are people who say that, you know,

after 20 years we've not sold everything with string theory, therefore it's no good. I think that's a terribly pessimistic view because, you know, why should you be able to solve the mysteries of the universe in 20 years? We might have to wait 200 years, but it's too soon to say it's not part of the legitimate science. I'm personally very confident that quantum gravity can be tested,

that we'll discover what the unifying theory is that incorporates generativity in quantum theory. And I have a deep confidence in the unity of physics, and I think that will play out, and we will know whether there are other universes, whether the multiverse is real. Martin, finally. Well, all I'd say is that we have to be open-minded about the options,

but unless some physicists are optimistic, they won't even try to find the answers, and unless they try very hard, they surely won't succeed. Well, thank you very much to Fadaka, Martin Rees and Bernard Carr, and thank you very much for listening. Next week we'll be discussing Kingley.