Instant Genius - The multiverse, with Lord Martin Rees

Episode Date: April 13, 2023

The idea of the multiverse, a hypothetical group of coexisting multiple universes, has long been a staple of science fiction books and movies but the theory is actually grounded in bona fide science. It has been gathering momentum amongst cosmologists for several decades but what exactly does the theory say and what evidence is there to back it up? In this episode, we’re joined by the Astronomer Royal and fellow of Trinity College Cambridge, Lord Martin Rees. He tells us about his thoughts on the possibility of the existence of the multiple universes, what parallel universes might look like and how our universe evolved to produce the ideal conditions for life. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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a bite-sized masterclass in podcast form. I'm Jason Goodyear, commissioning editor, BBC Science Focus magazine. The idea of the multiverse, a hypothetical group of co-existing multiple universes has long been a staple of science fiction books and movies. But the theory actually is grounded in bona fide science. It's been gathering momentum amongst cosmologists for several decades, but what exactly does the theory say and what evidence is there to back it up? In this episode, we're joined by the Astronomer Royal and Fellow of Trinity College, Cambridge, Lord Martin Rees.

He tells us about his thoughts on the possibility of the existence of multiple universes, what parallel universes might look like and how our universe evolved to produce the ideal conditions for life. Okay, so we're talking about the multiverse, and this is a concept that's much beloved by science fiction writers and filmmakers, but in fact it's a bona fide scientific theory. So before we take a deeper dive into this idea, could you just lay out what the basic idea of the multiverse is for us, please?

Yes, well, I'd say that physical reality, in our conceptions has been getting bigger and bigger. If you think back a few centuries, then we had the idea that the Earth was the center of the universe. Then Copernicus gave us the picture that the sun was the center of the solar system and the Earth was just one planet. And then in later centuries we came to realize that our sun was just a star and we realized that many, many stars and that our Milky Way galaxy,

just one of billions of galaxies which are in the observable universe. And we have this picture of our universe which is expanding. And that's about 13 billion years ago. It was in a very hot, dense state, and it's cooled down and formed galaxies, etc. And we'll go and expanding for a very long time, perhaps forever. So we are in this aftermath of the Big Bang, as it were. But the next step is that perhaps our Big Bang wasn't the only one. Perhaps there's another Copernican revolution beyond the one that's reduced the size of the Earth

compared to our concept of space. Perhaps there are many big bangs. And this is the concept of the multiverse. And there are many possibilities for a multiverse, which is great fun to discuss. So you mentioned there the Big Bang, just for the listeners that aren't quite clear and what that is. Can we just briefly explain what that is? Yes, indeed. If we look at galaxies a long way away, then they are moving further away from us all the time. They're receding. The cosmos seems to be expanding. And so if you reverse time and extrapolate back, then that implies that

everything was closer together in the past. And if you extrapolate back to a time which we can now pin down to be about 13.8 billion years ago, everything would have been squeezed on top of each other, very, very high densities. And there is evidence that the universe did indeed start off in a very hot, dense state. The main evidence comes from something called the cosmic microwave background, The fact that all of space is full of microwaves from all directions, which are the afterglow of the hot dense beginning of our Big Bang. Everything was hot and dense, hotter and dense in the center of a star. It expanded and cooled, but this radiation is still around.

It fills space. It's got nowhere else to go, and it's now these microwaves pervading all the space. So you've got fairly good evidence as our universe started off very hot. dense by 13.8 billion years ago, how far back can we extrapolate? We can extrapolate back with some confidence to when the universe had been expanding for just a billionth of a second, a nanosecond. And the reason we can't go back, you might say, why can't we go back into the first nanosecond? Reason for that is that in the first nanosecond, every particle had more energy than we can produce in the biggest accelerators like the large hetero and collider

in CERN in Geneva. And so we lose our foothold in experimental physics. We don't know what the conditions were. But after the first nanosecond, things had cooled down enough that we do have some evidence that the universe did come from that dense state. So we can confidently go back, not due this early stage. And that is tremendous. Because when I was a young scientist just starting, it wasn't clear that there was ever a big bang at all.

There was an idea called a steady-state theory that the universe existed from everlasting to everlasting, always in the same condition on average. But we do think there was a big bang. Now, it might seem it's not much of a loss not to be able to discuss the first, none a second, but it turns out that many of the most important things that happen, and laying down how the universe is expanding, what it consists of, etc., in terms of matter, radiation and mysterious dark matter and all that. All that was determined in this first tiny, tiny fraction of a second.

And so it's a bit of a mystery how the universe really began and was really a beginning. But to link to the subject of the multiverse, one of the issues is whether right back at the very beginning, at a time when our entire universe, our entire observable universe, was squeezed to the size of the tennis ball, and towards a perspective, when the universe was a nanosecond old, it was about the size of the solar system, but going about much, much, much further. And at that time, the physics was very exotic, but some ideas about the physics suggests that not just one big bang would form, but lots of others would sprout. There'd be a whole lot of big bangs forming,

and the main proponents of this idea is a business called Andrelinda, who has the idea what he calls eternal inflation, which is that the universe is getting bigger and bigger, but that's not just more galaxies, et cetera, but more and more big bangs. So this is a further Copernican revolution, extending still further our concept of how large physical reality is. So that's briefly why we talk about the possibility of other big bangs apart from our own.

So you mentioned that you called it like you compared it to the ideas of Copernicus, which is really interesting. And you mentioned that earlier in your studies they had this idea of the steady state theory. So sort of when and where and how did this idea of the multiverse originate? Well, there have been speculations in science fiction, of course, going back a long time. The fact that we can talk seriously about the early universe really has only been the case for the last 40 years. But in the last 40 years, there have been lots of speculations about the very beginning

in this first tiny, tiny fract of a second. And then people realized that one possibility is that the physics would allow not just one, but many big bangs to occur. It's been talked about for 40 years. It's still speculation and may always remain so. but it's certainly consistent with some popular ideas in physics. So you mentioned Big Bangs. In this theory, do all of the universes within the multiverse have to start with a big bang?

Well, in the particular theory they would. But as you say, there are other theories of sort of parallel universes, which are not quite like that. It's rather like, if you imagine, a population of ants on a sheet of paper and another population of ants on a parallel sheet of paper, then if those sheets of paper were infinite, then the ants were in infinite universes, and they might not be aware that there was another population of ants

in a parallel universe, as it were, and one dimension up, then there are some theories which say that we are in our universe with three dimensions of space, but just a tiny distance away from us. there could be another three-dimensional space separated from us by a small amount in four dimensions. That's an example of another kind of multiple universe. But the most popular idea is the idea called eternal inflation, which is many big bangs all originating from hot, dense beginning.

So you mentioned that eternal inflation. So that's this idea of cosmic inflation. So what exactly are we talking about when we're talking about inflation? Well, inflation is the idea that the universe expanded very fast initially and got very big. There are fairly good reasons for taking inflation seriously. It does explain certain phenomena we can observe in particular some details of the microwave background radiation. So the idea of inflation is taken seriously by a lot of people. But it's only some variance of inflation which allow the multiple big banks.

The so-called turn-inflation is a particular idea, and whether it's right or not depends on the appropriate physics at that time. And the trouble is that we don't know the physics at this very extreme state when everything was hugely compressed, essentially because it's far for many conditions that we can reproduce in the lab. So I think one concept that's sort of baked into the idea of the multiverse, at least as I understand it, is that it in some way provides an answer to the question of why our universe is so finely balanced that it has the conditions necessary for life. I mean, is that, what do you think about that?

Is that an argument that you find compelling? Well, I mean, I think it's quite compelling in that there are some apparent fine tunings of our universe. It's exaggerated, I think, by some people, but it's easy to imagine universes. which are counterfactual and in which you can't imagine any complexity evolving. As we could talk about one or two of these. The question then, though, is if there were these other big bangs, would they all be governed by the same physical laws?

Would they all have the same gravity, the same masses for the atoms and all that? So would they all cool down to a universe that was rather like ours? and we don't know that's possible but it's also possible and that much more interesting that they could be different I mean the most obvious differences is that they may start expanding at different speeds

so some may for instance collapse quite quickly and then of course there's been no chance for anything complicated to happen or some might expand so fast that gravity never gets a foothold and everything just flies apart and those stars or planets can ever form. But even if we have a universe which is able to somehow form things like stars,

then we can imagine that they be very different if some of the other important numbers in physics were different. For instance, gravity is a crucial force, of course, in holding us down on the earth and holding stars together, etc. But it's a very weak force. It's a weak force in a sense that if you take two atoms and hold them close together, to protons or something like that, then there's an electric force between them, but it's also a gravitational force between them.

But the gravitational force between two atoms is about 40 powers of 10 weaker than the electric force. And so if you're a chemist thinking about the structure of molecules, you think a lot about the electric forces between them, positive and negative, etc. Because they determine its structure and whether it's a bound molecule or not, which you don't think about the gravity compressing the whole molecule. But of course, if you imagine building up bigger and bigger structures, then electricity has both positive and negative particles, the electrons and the protons, they more or less cancel out.

So the electric force on any big objects more or less cancel out but gravity as it were always has the same charge it adds up so if you imagine building up bigger and bigger structures

let's start with a sugar lump then a human being and then an asteroid then gravity is getting stronger but it's still not very strong but when you get to something big as a planet then gravity is strong enough to make it round

and when you get up to the size of Jupiter it's strong enough to make the planet round, but also starts squashing it, so the centre gets up for high densities. And then something 10 times bigger than Jupiter gets hot enough to become a star. And so in very big objects, bigger than Jupiter, gravity is able to dominate and crush them. And all stars are in a state where it's a balance between the pressure of their hot interior and the gravity. and so gravity is important then but the key point is that because gravity starts off with this handicap of 10 to the power 40

then you've got to have a very big object in order for gravity to actually win and that's why planets are very big compared to atoms because they've got to be big enough that the gravity actually overwhelms the other forces and so if you could imagine a counterfactual universe where gravity wasn't so weak and you do the thought experiment of building up sugar lumps and human beings, etc., then things don't need to get so big before gravity

starts crushing them. And it could be stars in this hypothetical universe, but in a sense of gravitation-in-bound fusion reactors which would shine, but they'd be much smaller. You wouldn't need to act such a big set of atoms together in order to get a star. So stars will be much smaller and much less long lived. And so I think we can say that if we had a universe where gravity wasn't as weak as it is, then you would not be able to have as much space and time as is needed for evolution of life to happen. you find that the stars didn't live very long, objects as big as us would be crossed by gravity, etc.

So although gravity is not finely tuned, it's got to be very weak. And so that's one example of a basic number, the strength of gravity, known by the capital G as the physicists, why that number must be a small number compared to others. But you could imagine changing other things. If you think about the atoms themselves, then the fact that atoms bind together to make the elements of the periodic table, helium, lithium, carbon, oxygen, all the rest. That depends on a balance between two different forces, the electric force which tries to push all the nuclear part, and another force called us the nuclear force, which packs them together. If there wasn't a fairly close balance between those two forces, you wouldn't have the elements, the famous periodic table, of a hundred-old elements. And we might, for instance, just have hydrogen.

In fact, I wrote a little paper recently about what I called the nuclear-free universe. This was a universe where gravity was the same, and so stars could form, but there'd be no nuclear energy. there'd be just hydrogen. And if it was just hydrogen, it could still be stars, but there'd be no interesting chemistry and therefore almost certainly no complexity and no life. So if you looked at that universe, then I'd like to say it would bear the same relation to the actual universe

that a marble statue does to a real human being. You'd still have things which look like stars and galaxies and even giant planets. but they're just made of hydrogen. And this is an example of where there's a tuning between the so-called nuclear force that holds complex nuclei of elements like carbon and oxygen together

and the force of electrical force which drives them apart. And incidentally, I think I should mention I think one of the most beautiful ideas in astronomy, which is the ideas about how the atoms we are made of were formed from pristine hydrogen. Within a big bang, the emergent material doesn't contain any oxygen or carbon or iron or anything that we are made of.

That material is all made by nuclear synthesis in stars. When a star is shining, it's getting its fuel by nuclear fusion, turning hydrogen to helium and then helium into carbon and then carbon into oxygen, etc. And when a big star ends its life by running out of fuel, it exposes a supernova explosion

and flings back into space the debris which it was made of. The inner part may collapse to be a neutron star of black hole but the outer part is flung off and that material will go into interstellar space

and then it will form in its interstellar cloud and maybe condens into a new star, because stars in our Milky Way are forming all the time, and they're dying all the time. And so in our galaxy, a star like the sun is not a first-generation star. It formed from gas in its space already contaminated by the debris from the earlier generation of stars which had lived and died.

And so we have the marvelous idea that we are linked to the stars in a more intimate way than even the astrologer's thought, in the way we're made of lashes of long-dead stars, with a less romantic with a nuclear waste from the fuel that made stars shine. And indeed, this idea was first suggested at 60 years ago by Fred Hoyle, who was my predecessor as Professor of Cambridge. And he first realized this, and it's been worked on ever since. And we can now say which kinds of stars produce particular chemical elements, and also why oxygen and carbon are common, but gold and uranium are rare, but how these all came to be in our solar system. So this is a wonderful story. And of course, because of these processes that we do have now, all the chemical elements of which will live, living things are made. And if you tuned the nuclear force differently, none of this could happen

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So when we're talking about the universes within the multiverse, so is there an infinite number? And how do we know that? What's the thinking behind that? Well, of course, we will never know the difference between infinity and Velaar's number. There's certainly thinking which suggests, even if we take just our Big Bang and its aftermath, that could be a lot more extensive than what we can see, because when we look at distant galaxies, they're receding from us, and at a certain distance through tens of millions of light years,

beyond that, they disappear over the horizon, which is actually rather like an inside-out version of what happened if things fall into a black hole. In the expanding universe, where we now know the expansion is actually accelerating, then distant galaxies will eventually accelerate away and disappear from view, to fall over horizon. But that's not the edge of everything any more than if the ocean, the horizon around you is the end of the ocean. And so most people accept that the aftermath of our big,

bang extends a lot further than we can see. So there's lots of unobservable stuff, unobservable galaxies beyond the horizon. And so that, which is quite good arguments, indicates that the universe is bigger than the observable universe. But then, of course,

that's just the aftermath of our big bang. If there are other big bangs, then there's far more still. And none of this is ever going to be in principle observable. the if galaxies are accelerating away from us, they'll never come into view if they're already expanding a wave beyond the horizon. And ditto, we can't have direct evidence for the other big bangs. And so this will never be more than a theoretical concept. But that doesn't mean

that it's metaphysics. It's now speculative science. And we might one day have a theory, string theory is the most popular idea now which can describe the physics in its first tiny tiny fractal of a second when the laws are set up and if we had such a theory which could be corroborated by observations we can make in our universe

in our low energy world then we would take seriously its other consequences even when we can't observe them It's a heresy to think we've got to be able to verify observationally all the consequences of a theory in order to take them seriously. Let me give you another example. We take quite seriously what Roger Penrose and people tell us about the inside of black holes, even though we can never observe them, because we've got a theory, general relativity, which has been tested and vindicated in other contexts. So because we know the theory works in many contexts where we can test it, we're prepared to take its prediction seriously even when we can't directly observe it, like the inside of a black hole. And likewise, we may one day have a theory which unifies gravity in the other forces and which explains many things we can observe.

and if that theory predicts the idea of the multiverse like Andrei Lindy proposed, then it will be a reason for taking it seriously as a possibility, even though we'll never be able to observe them. Yeah, so it's sort of following on from that. You said that the idea has been sort of gathering momentum. So how sort of popular is it at the moment? You know, what sort of proportion of cosmologists supporting? it, you know, behind it?

Well, of course, one shouldn't take the majority of view as being the right view. It's certainly taken seriously by a number as a possibility. I mean, I think if you ask me, then I'm open-minded. I don't think we've missed this. It's a speculative idea based on serious assumptions. It could be correct. And so I think we should take it seriously. But it'd be a long time before we can actually understand.

understand this better because string theory, incidentally, is a theory which says that space is much more complicated, what we see. We think of spaces, three dimensions and points, but according to string theory, every point in our space, if you magnify it enough, is a tightly wound origami in five more dimensions. So the space is very, very complicated, and the critical scale is much, much smaller than atoms and so very hard to check. I think, incidentally, that the mathematics of string theory, which is geometry in ten dimensions,

is the kind of thing. You may have to await computers, like, well, deep mind could play chess and go much better than a human, and likewise there may be machines which, given the rules and the axioms of this ten dimension of geometry, might be able to work through its consequences

in a way no human mind ever could. and if the machine sputes out at the end something true about the standard model of low energy physics, then we take it seriously. You wouldn't get the aha insight of having understood the theory, but we would know that it had some validity and we would take it seriously. So that's what might happen in this theory. At the moment, it's a perspective given. It's become a bit more serious, and I think I organized,

two conferences, one in 2001 and one in about 2008, on this sort of thing. And certainly the second one was taken more seriously than the first one. And to give you another anecdote, I was on a panel with And Delindian and some other people, and the chairman of this panel asked us at the end, the question you asked me about how much would you bet on the multiverse? and the chairman asked, would you bet your goldfish, your dog or your life? And I said, I was nearly at the dog level. Linday said, well, he'd spent 20 years working on his model,

so he almost met his life on it. So he took it very seriously. And when this story was told to Stephen Weinberg, one of the greatest theoretical physicists of his era, he took quite seriously. he'd happily bet Martin Wies's dog and Andreda Lisman, understand understanding being correct. Some people think this is something to be poo-poo.

There's not real science. But I think it's just one of the many, many things in science, which we can't yet understand and maybe will be forever beyond our understanding because just as a monkey can't understand quantum theory. There may be deep aspects of reality which the human brain can't understand. So do we have any theories on how

these separate universes are somehow, I don't even know if simultaneous is the right word, how they're able to coexist? Well, I mean, it's not clear how they do coexist because there's no common measure of time between the different ones, time in each one. I think all you can say is that they're all part of physical reality in these models, and that's the case. But one thing I should have said is that many versions of string theory do have the idea that there are many different kinds of empty space, different vacuum states, and in each of those, the laws of microfiche are different. So the idea of having different atomic physics and different strengths of gravity, etc., which makes the not a big band evolve differently and not just be duplicates of each other, that is an idea which is a, implicit in most versions of string theory. There are lots of different vacuah.

So that's concerning the birth of the multiple universes? The way they cool down, because if they cooled down like our universe did, they won't end up on this picture being governed by the same laws, particles of the same masses and all that. And so some of them may become what I call the nuclear-free universe, where there are no chemical elements, and some of them may have a gravity that's too strong for stars to be long-lived, etc.

So you mentioned that our universe, obviously, has the conditions perfect for the evolution of complex life. So surely if we have an infinite number of other universes, it's possible that complex life forms exist within the other universes? Indeed, and of course we can't say our universe is, In fact, I've argued that if gravity was 10 times weaker, it actually is, this would be even better because then stars would last longer, they'd be bigger. Planets and human beings and things could be bigger without being crushed by gravity. And so, more space and more time.

Our universe may not be quite the optimum, but it's in the group that does allow complexity, whereas there are some, like that I mentioned, where is most unlikely as any complexity. What we can't yet do is sort of put any measure on the likelihood of different types of physics in different universes. Often what you see in science fiction films is people travelling between different universes within the multiverse, and that really is in the realm of fiction, right? Well, I think they normally do this in the wormhole, don't they, or something like that? And of course, there are some models where there is a wormhole, which looks like a black hole,

which we go into it, then you come out. So we can't be, we can't be sure that that's impossible. But to come back is, I think, unlikely. So just as like a slight tangent then, so how does this idea differ from the so-called many-worlds interpretation of quantum mechanics? I think a lot of people might think that they're two sides of the same coin. No, they are different. I mean, they're both ideas, but you can have one or the other, or you can have both, really, because many worlds of the population is the idea that

when the universe, when anything happens, which has two options, then, in fact, the universe bifurcates. and both options are taken and we find us in one than not the other. This is, again, a logically consistent model. This, of course, is even more complicated, really, because if every time anything happens, the universe doubles in size,

then within a fraction of a second, you would have zillions of different parallel universes. So this is actually even further from common sense than the kind of multiverse that I was talking about. But again, it's a way of thinking of quantum theory and the mysteries there, which is also taken quite seriously. And I think I would say that even though it does seem an extraordinary idea, we've got to realize that it seems extraordinary to us

because our brains haven't changed very much since 50,000 years ago, our ancestors Rome, the African savannah, and a cup of the everyday world. And it's rather amazing. We've got as far as we have done in understanding the micro-world of particle physics and the grand scale world of the cosmos, where we don't expect our standard intuitions to apply. Yes, I think the message really then by way of summing up is that we need to keep an open mind. I think an open mind on things like that. And, Of course, the question is whether physicists will be able to probe these mysteries. They're clearly making progress all the time.

But I think we do have to be open to the idea that there are some mysteries which humans will never be able to understand. Just as I said, monkeys can't understand quantum theory. But of course, in the cosmic perspective, then we shouldn't be too depressed even by that. unless we are sort of human chauvinist, as it were, because one thing that we know as astronomers is that we humans are the emergence from 4 billion years or so of Darwinian evolution from the first protozoa in the young earth. But although many people think that we are somehow the culmination at the top of the tree,

astronomers know that the future is longer than the past. the sun has got six million more years before it dies out and the expanding universe may go on far, far longer than that, almost forever. They like to quote Woody Allen, who said, eternity is very long, especially towards the end. We've got to bear in mind that we are not the culmination of intelligence. The big question, really, which fascinates many of us, is there other intelligence out there already?

or it is going to be our remote descendants, which are of these problems, and also will there be a transition from flesh and blood brains to electronic brains? I guess there may be limits to the power of the sort of brains that organic entities like us have, and maybe the machines will take over in the far future. That was the Astronomer Royal, Lord Martin Rees. Thank you for listening to this episode of Instant. Genius, brought to you from the team behind BBC Science Focus magazine. The current issue of BBC Science Focus is out now.

Pick up a copy wherever you buy your favourite magazines or download a digital copy from your preferred app store. You can, of course, also find us online at sciencefocus.com. This podcast is sponsored by Name, Audio and Focal. The texture and emotional depth of music can be lost through digital sources or poor signal. Name Audio believes you can have digital precision with Analog warmth. Alongside French acoustic specialist vocal, Name creates high-end audio systems, combining innovation with craftsmanship, so you can listen to music, just as the artist intended.

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