The Infinite Monkey Cage - Does Time Exist?

Episode Date: August 26, 2020

Brian Cox and Robin Ince are joined by actor and writer Mark Gatiss, theoretical physicists Carlo Rovelli and Fay Dowker to ask timely questions about time. Is time real, does it exist in the fundamental laws of physics, and if it doesn't, why do we experience the sensation of time passing? They look at the idea of the block universe, where our future is as real as our past, which worryingly leads to Robin's favourite question about free will...is that an illusion too? A timely look at the question of time and hopefully just in time...Producer: Alexandra Feachem

In our new podcast, Nature Answers, rural stories from a changing planet, we are traveling with you to Uganda and Ghana to meet the people on the front lines of climate change. We will share stories of how they are thriving using lessons learned from nature. And good news, it is working. Learn more by listening to Nature Answers wherever you get your podcasts. This is the first radio ad you can smell. The new Cinnabon pull apart only at Wendy's. It's ooey gooey and just five bucks with a small coffee all day long. Taxes extra at participating Wendy's until May 5th. Terms and conditions apply. BBC Sounds. Music, radio, podcasts.

Hello, I'm Brian Cox. And I'm Robin Ince. This is the Infinite Monkey Cage. And today, I suppose, well, we should start with one of the great quotes of physics, time is an illusion. And of course, that was said by Ken Dodd when he was playing the Bradford Alhambra back in 1972. It was just before dawn, it was the beginning of the third act and all the Diddy men came out dressed as Einstein and started singing tears. It was a beautiful moment where physics and variety met. Right, for our many listeners outside the UK and or the 1970s, Robin's introduction will need some additional cultural context. So much in fact that we need more time than we have. And in any case, to be complete, we'd have to know what time is, which is coincidentally the subject of today's

monkey cage. But of course, I know what it is, because there's the great famous quotation from physics, which is time's an illusion, which was first said by Ken Dodd at the Bradford Alhambra. It was just turning towards dawn as the Diddy Men came on for the beginning of the third act, singing tears dressed as Albert Einstein, and I think also Paul Dirac. I'm so sorry, I seem to have got caught in a certain part of the block universe.

So, moving on. I think that's explained everything. I feel the Ken Dodd illusion means that we don't require doing the rest of the show, so now we'll just play some music until 9.30am. It's the idea of the Diddy Men dressed as Paul Dirac. I know, it's a beautiful image, isn't it? Dirac Diddy Men. Here come the Dirac-y men.

The Dirac-y men. This is, I'll tell you what, for those of you who don't normally listen when we're on later on in the afternoon, hearing this in the morning, it's very different to Melvin Bragg, isn't it? Today, to discuss our current understanding of time, we're joined by two physicists and a writer with a penchant for manipulating time. And they are... Hello, I'm Carlo Rovelli. I am a theoretical physicist. I work on quantum gravity. And the most disconcerting thing I find about time is that there is no global present, which means that if I ask what is happening here now,

I know, I'm just talking, but if I ask what is happening now in a distant galaxy, the correct answer to the question is that the question makes no sense. Hi, I'm Faye Dauker. I'm a professor of theoretical physics at Imperial College in London. And the most frustrating and disconcerting thing about time that I find is my inability to convince my colleagues that time is not an illusion, that it is physically real and not just a figment of our imaginations. Hello, I'm Mark Gatiss. I'm an actor and writer. And the most frustrating, disconcerting thing about time is that I'm still unable to control it despite many attempts. And this is our panel.

Carlo, before we start, time is, I suppose, something that everybody thinks they know about. It's a very visceral subject, isn't it? We all age. Time is central to our lives. So why is there a problem in understanding what time is? I think to some extent it's precisely because it's so visual and so emotionally significant to us. We cannot just talk about time in a detached way because we live in time.

And time also brings us everything and takes away everything from us. So it's a source of our, you know, the impermanence of the world. And so every time we talk about time, we are touched inside ourselves. And time, it has,, we are touched inside ourselves. And time, it has, time is a very complicated thing. There is an experiential, emotional experience of time, which of course is not in physics, depend on our brain, our own way of thinking, made by memories, expectation. And it's hard to separate that from the actual physics of clocks and the physics of passing time and the difference between the past and the future and all those kind of things. So it's a complexity of time, I think, that makes it an interesting subject, but also a subject that

touches us. But it would seem, Faye, that even as Carlo says, we have this such a personal experience of it that it's perhaps hard to detach ourselves from it and study it dispassionately. It would seem, surely, that we can agree that it exists. But as you said, you have a problem convincing your physics colleagues that there is such a thing as time. So could you say why there's a problem there? It's an extraordinary situation. So could you say why there's a problem there? The situation has arisen largely because our current best scientific understanding of time is general relativity or within general relativity.

That's Einstein's theory of gravity. And Carlo, in his introduction, touched on a very important thing that general relativity has taught us, which is that there's no global now. There's no global present moment, only a local present moment. There's no global present moment, only a local present moment. And that makes it very difficult to situate anything like a passage of time within the theory. In fact, people have basically given up. So the picture of the universe that people, most scientists have within general relativity, is what is often referred to as the block universe.

within general relativity, is what is often referred to as the block universe. And that's a picture in which all events, past, present, if they exist, and future events, they all have the same status. They're just as real as each other. So if one believes that past events have happened and are real, then future events are just as real. Or if to use tense language, you could say that the future has already happened. And in this block, all of these events are laid out once and for all in a timeless way. And there's nothing in the theory to coordinate

with that deep fundamental aspect of our experience which is its temporality the fact that we have experiences in time we experience one thing after another mark how do you react when you you hear those things like when we hear about different ideas of the possible kind of fictional nature of time when we hear about the block universe so we're five minutes into recording this show and yet the end of this show exists already it's we we have an experience but it's there and it feels like that seems so counter instinctual to to kind of say there is another your reality is some kind of falsehood though we can never actually jump away from it or can we ever escape from that reality you know what's very strange is in such an intense scientific discussion it sounds a lot like destiny doesn't it? If you have a block pavement stretching out in front of you

which is absolutely impossible to change that sounds that's like something that the ancient Greeks would nod their heads sagely about say ah well don't you see? I mean it's such a fascinating idea and we've all you know as Carlos says we're all so caught up in our own version of it, our own, you know, we know that our tea gets cold, we know that our hair falls out, we know that people die, things die, everything grows old. and sort of project yourself forward and think, well, that's happening now. You know, Henry VIII is currently divorcing Catherine of Aragon whilst Donald Trump is losing his second election, say. All these things are rolling along at the same time. It's just extraordinary.

May I ask, though, is that quite the same as a multiverse? Does that mean that literally everything possible is happening at once or there is a certain yellow brick road which is just the way it's going to be can i also just mention by the way for for those of some of the audience can actually see brian when you said we all know our hair falls out brian gave a very superior look by the way just uh um but yeah the the multiverse idea i mean in fact i don't even think we should go into the multiverse because the idea of getting two physicists... We already have.

No, it's a very different story than the multi-universe. The multi-universe, it's a speculation of some people that some people may like, some don't like. It hasn't been, in any sense, proven a useful way of thinking about the universe yet. Here we're talking about general relativity. So here we're talking about a theory which is 100 years old, which has given an incredible amount of predictions, all confirmed,

which we use for the GPS in our car. So it's totally solid science, general relativity, which tells us that temporality works different than our intuition. And I think that's the main message. And it does so in a variety of ways. For instance, it tells us that it's perfectly possible to go for a trip, come back and find, when we're back, find our children who are much older than us. And this is just shocking, the movies about that, but it's true. And so we have, in a sense, to reorganize our way of thinking time to allow for these true facts of the world to be possible, to be real, to be actual. We believe that they are, nobody questions that

in physics. So I think that the core of the story here is that time is different than the way we think about it usually. There's no doubt about that. We can, you know, go for a trip and come back and find our children older than us. So therefore time is different than the way we think. Time is not an illusion. It's not at the fundamental level the way we think it is. Faye, if you ask most people, as in non-physicists, non-scientists, you ask them what time is, they would probably think they knew exactly what it was. Time is the thing in which things happen. That's it, you know, that's it. So when does it become, at what point of understanding, at what point, what issues come in where you start to go, well, actually,

time just as that is not enough anymore. It's not enough just to see it as this continuum. I think this aspect, this local aspect of time, the experiential aspect, which has been referred to, it's telling us something fundamental about future physics because, as I said, it's not captured within general relativity. There's no place for it in general relativity. The most natural picture of not captured within general relativity. There's no place for it in general relativity.

The most natural picture of the world in general relativity, which is currently our best theory and super successful and hasn't been superseded yet. But I think that our experience is telling us something about future physics, about a theory that's better than general relativity, something that will explain general relativity, something that underlies it. And I think that our experience is a clue. It's telling us a direction to go in, in doing future research, which is quite amazing, because it's something that everyone has direct access to. And yet it's subjective. That word has already been used, people have already brought that up. And that's the key to why it's so difficult to discuss,

because the question is, do we take that subjective experience as data that needs to find a scientific explanation? And some people just say no, it doesn't, because it's subjective. But I take it seriously as empirical evidence of something. What that thing is, we don't know. Mark, it's a remarkable idea, isn't it, that that thing that we almost hold most dear, the passing of time, the fact that we were children

and we will be old and one day we will die, is somehow not part of the fundamental description of reality, perhaps, which is certainly Carlo's view. I agree with Robin, this is complicated. I mean, it's a fascinating sort of grey area where science really does become like philosophy, doesn't it? I can imagine this discussion being had in 15th century Florence in a way in the sense that it's so, it's like, well, what are we?

What on earth are we? It's the wilder realms of sort of speculation, isn't it? If what we believe to be the absolute fundamentals of existence, we're born, we grow old, we die, we fall in love, we eat, we drink, we do things, are actually just an entirely a kind of construct, then we are absolutely, you know, focus, joining science with art. We're asking the most fundamental question about ourselves. I think that's intriguing. But isn't this science what science has done in the best so many times? I mean, with the Copernican Revolution, with Darwin in antiquity

when they figured out that the Earth was round. I mean, we are burned thinking that there's up and down and then we discover that it's a local thing. We grow up thinking that the's up and down, and then we discover that it's a local thing. We grow up thinking that the sun goes around us, and then we discover that, yeah, that's not contradicted by science, but it's just our perspective, and so on. These are not illusory things. They're real things, but they're complicated. They're not fundamental. But if, for instance, you know, the Darwinian revolution changed an awful lot of people's perspective up until the present day on where we came from.

But that doesn't alter the fundamentals of how we live our lives. if we believe that time is real and we are going to grow old and die, if we accept the concept that it's actually not, then what do we do on a day-to-day basis? What does that change apart from the fact that we can sit and scratch our chins and go, oh, well, good. Nothing really. It's not the same as actually discovering a fundamental truth about the universe, that the Earth orbits the sun. That's a real discovery.

This is like a sort of philosophical point, isn't it? Careful, you're going to lose them all their grants if you're not... Oh, we've just found out you've been doing philosophy. That's no grant for you. I want to throw that to Faye. I'd like to throw that question to Faye. Is it just mere... Does it matter? I mean, Mark... No, it does.

Mark is trying to suggest that there's a something that's just purely you know optional here we don't need to consider the problem one of the joys of working on quantum gravity is that you grapple with questions ancient questions so you mentioned renaissance florence or something i mean this question has been debated ever since we have records of there being debates about the nature of the world. So Parmenides was on the, what I would call block universe side of the debate, believing that, you know, that the universe just is. So that, you know, these things are, they're, they're really, really ancient, but it's not just philosophy. We don't understand a lot of things about general relativity, because we don't understand how it really interacts with quantum

matter. So matter curves space and time, space-time. But we know matter is actually quantum mechanical, and we don't know how quantum mechanical matter curves space and time. So we want to make progress in understanding space-time, in understanding gravity better, understanding black holes better, understanding the final fate of a black hole better. And to do that, we need to make progress in fundamental physics that will be tested. It will be empirical. It will have consequences that we can go out and test. But how do you do that? What guides you in pursuing a particular direction of research in this area? And these sorts of things are guiding one to do research.

So it's really important that we have a diversity of perspectives on on our current theories in order to to have the best chance of making progress. to have the best chance of making progress. The ground is safe. But I see what Mark means, because there is something about, whether it is about us going around the sun, whether it is about our connection to all other living creatures through natural selection, there's a kind of picture theory of language.

You can see the picture of it. And yet there is something strange, however practical and useful, and all of those different things that these ideas are and however wonderful they are. Somehow you are not then you can't. But can you build on that for your reality, for your personal reality? I'm not talking about for the importance of scientific research. That bit where you go, however much you understand time does your your relationship, you are not then able to go,

well, now that I've found out time is an illusion, this is the way that I'm going to be able to catch that bus that I missed or whatever else it might be. Do you see? I don't know if that's a long way... People who believe in the block universe will have to speak for themselves. But for me... They've already spoken. So I believe the past is real, but the future is not,

that it's open and it hasn't happened yet. See, that's the hardest bit, I think. That does affect my day-to-day feelings. I mean, the past is real. For example, the past is real. The events of the past are fixed and real. They influence me. That has some, you it's it has both con

i find consolation in it in terms of in terms of people that i've lost but i also find it's also hard because things that you did that you regret they're real and and concrete and and cannot cannot be erased they're they're physically real too so these ideas for me they're not and concrete and cannot be erased. They're physically real too. So these ideas, for me, they're not just abstract. They do impinge on me at an emotional level. Carlo, this discussion raises some very, I think, simple and direct questions. One is, we remember the past.

We don't remember the future. So what possible explanation can there be in a timeless theory for the difference between the past and the future? that originated in physics even before general relativity. Because in modern science since Newton and with Maxwell and Faraday and general relativity, special relativity, quantum mechanics, quantum field theory, it came as a surprise, of course, that the fundamental laws of physics seems to be symmetric if you change t minus t. So there's a question of how come that our experience is so dramatically non-symmetric in past and future.

But of course, there is a law of physics which makes the distinction because of the second law of term X, because the heat goes from hot to cold things. Now, the second law of term dynamics, this thing we understand very well. We understand why if you have a cold and hot thing, heat goes from the cold to the hot and not vice versa. And in a sense... Just to be very clear, just to be very clear. So you're, for example, saying that ice melts. Yeah.

So that gives us a direction. Ice melt gives us a direction of time, which means that if you start in one direction of time, which we call the past, from ice and water, and you leave them there in a hot day, you know for sure that ice will melt. So this we understand. We understand that if there is this initial dissymmetry due to the fact that you start with ice and water, physicists say low entropy, then the ice will melt.

So physicists will say entropy goes up. Now, once we have understood that, is that sufficient for giving us an understanding, accounting for the fact that we feel the past is fixed and the future open? I think this is a beautiful scientific question. I think it is sufficient. And in fact, I've worked on that. question. I think it is sufficient, and in fact I've worked on that, and I think that one can argue in general that in a world where the entropy goes up, so in a world where the second law of thermodynamics holds, where the entropy was low in the past, automatically there are a lot of

traces of the past that exist, but not traces of the future. So the thermodynamic dissymmetry has a consequence that today, you know, there are steps in the sand that are correlated to somebody walking, there are craters on the moon which are correlated to rocks, meteorite falling in the past, but there are not similar things about the future. In our brain there are plenty of memories of the past, but not anticipation of the future. And this existence of traces of the past, it's a reason, it accounts for our sense that the past is fixed and the future is opening. Not because it's illusionary. So what we mean by saying the past is fixed is exactly that. It's nothing else than that. In our brain, they're full of knowledge of the past

and absolutely not corresponding knowing about the future. Maybe I can ask Mark, that imagery of there being imprints of the past that exist, but no imprints of the future, is a wonderful sort of image isn't it it's a beautiful science fiction type well literally the first thing that came to mind apart from that tolling clock uh was um time the passing of the end that was was the brilliant uh there's a lost play by nigel neill called the road nig Neill, who created Quatermass, one of the great visionary writers of our time. And he wrote this wonderful play in the 60s,

which is now lost, sadly, which is about an 18th century haunting. It's a wood. And the local people believe that Bodecia's soldiers are tramping through it because they can hear footsteps and shouts and cries on a road where there is no road. And it's a wonderful play. It's a sort of philosophical debate between a sort of 18th century coffeehouse intellectual like Dr. Johnson and this local squire who does experiments using cat's whiskers and stuff like that. It's a beautiful piece of work but the brilliant brilliant twist right at the end is

when we experience the haunting I mean there are these flashes of light and sound and then suddenly we hear what they hear and it's a siren and suddenly you realize that the voices are people stuck on a motorway in the future and it's the epicenter of a nuclear explosion and it's a backwards haunting and that's exactly as soon as you said that but we we don't see memories we don't see imprints of the future that's the first thing i thought of course nigel neal got there first but that's a it's a it's a beautiful idea that actually there are ghosts of the future and and we are just we experience them in our own way see Nigel Neill must have been I was going to say just because now we're on let's keep it on Nigel Neill for a while Mark and then we'll get back to the

physics but it's um you've rubbed in something to say but no I just I just think it because he was always fascinated you know something like the stone tape as well and then then john carpenter's film prince of darkness which is influenced by him as well that idea of time being and i think again actually when we're looking for the practical use of thinking about the block universe this idea that we are standing in a kind of palimpsest of events upon events upon events i think does have quite a heartening sense in the kind of fragility of life that it is in some ways you can get a sense of it in the same way if you touch a stone and a stone circle you get there's a moment a kind of intangible moment where you feel for you know somewhere sometimes fed through those ideas of physics you get a sense of that we are still surrounded by many more times than our own time what we think about

you hippie no it's true i mean that's what you want to feel isn't it you i mean i go to i go to derham cathedral almost every christmas eve i'm i don't have any religious beliefs at all but to stand there and think that people have stood exactly there for a thousand years on that night doing the same thing is something intangibly wonderful about that. Just before we move on from this, this idea of the reason that there's a difference between past and future being, I suppose the analogy would be the reason there's an up and down in our experience is because of the presence of the Earth. Yeah. But in space, not fundamental. If you go out into space between the galaxies, there's no up and down.

I suppose in a sense what you're saying is the reason we remember the past and not the future is the presence of this thing in the past, which is the Big Bang. There's this strange, this perhaps origin to the universe. So that's the, it's hard to believe though, I'm sure for most people listening who are following along, I hope, is this idea that the only difference, that the future in your perspective,

not Faye, I'll go to Faye in a moment, but in your perspective, the future is there. There is a block universe type description. The past is there. The only difference between them is that, as you refer to the second law of thermodynamics, it almost feels like a technicality. But in fact, the whole history of the universe, past, present and future is mapped out. I mean, is that what you're saying? Is that you're saying that the only difference between past and future is this special state somewhere in the past? Yes, I strongly suspect that all the difference between the past and the future, which is

enormously vivid and strong for us, is just a consequence and can be accounted for of the fact that entropy is growing, if you will. And so there are traces of the past, there are memories of the past, there are memories of the future, in the same sense, as you're saying, in which the up and down are not fundamental, but we do have a clear account of its origin is the fact that we sit on a big planet, a rock that attracts us, so down is toward the big planet and up is the fact that we sit on a big planet, a rock that attracts us, so down is toward the big planet and up is the other direction. So it seems to me that we make a mistake if we take a complicated, a complex experience we have and think that this is fundamental in the world.

I mean, I have very vivid experience. I'm afraid of something. But fear is not in fundamental physics. Fear is in my own, it's not an illusion, it's true, but it's a complicated thing. So I think that all the difference between the past and the future could be accounted by the second law of dynamics. So, Faye, I want to give you the chance

to respond to this statement. I suppose it's the most widely accepted, by the way, isn't it, I guess, in theoretical physics, that the real difference, the only difference, is there's an asymmetry because there's a very special state as a cause of this highly

ordered thing in the past. That defines what the past is. Apart from that, there is no other difference. That's right. I think a lot of people have that view. It's not an obvious explanation. You have to do work. It's complex. So the proposal, the approach that I follow, the difference between the past and the future is a really simple thing i mean we we remember the past and don't remember the future because the past

has happened and the future hasn't happened this is a really it's a really really simple explanation it's you know we experience time passing because time does pass. It doesn't, I mean, we leave it to the neurologists to do more science about the way that our brains work. But the hypothesis is that we experience time passing because there is such a thing as a physical passage of time. So there's something there for us to experience, how we experience it and in what way and whether we if we're afraid whether we have the same experience of time passing as you know if

you're just sitting there chilling in the garden with your you know with your with your drink you know that's for psychologists to to figure out but um just to bring it's really really simple well i was just thinking that we all i want to say, we always have this argument at the end of a discussion on Monkey Page. We usually ask a question, so is there such a thing as free will then? Yeah. And it's a very simple question.

But I suppose, let me characterise this and you can tell me that I'm wrong, Faye. In your picture, there would be such a thing as free will and in Carlo's picture, there would not. I don't think that the distinction between carlo's point of view and mine impinges on that question it all depends on what you mean by free will yeah that's the thing so call in melvin if you mean something extra

physical something that's beyond the material then no both carlo and i'm saying no there's no such thing as free will in that sense some you know some soul or some i think i mean correct me if i'm wrong carlo so but if you think there's is there a reason for everything that happens well some things that happen there's a reason. I mean, you know, there's cause. Some things that happen are just random. I don't think this way. Just to simplify, in a block universe picture, then my death exists as an event. It is there.

It's got coordinates on this space-time manifold or map. Whereas in your picture, it does not exist yet. It doesn't exist. So that's correct. Carlo has planned your death, Brian. That's the difference. He's worked it out to the minute. Brian, I'm surprised because usually every time you say something, I agree 100% what you say. This time I don't. And in fact, I fully agree with Vei. Let me put it this way. You said in Carlos' picture there is no free will. Of course there is free will. I mean, I am free. You're free.

It's just what do I mean by free will is that there is something that decides. It's me. It's my brain. It's things that are happening. I don't have chains. I'm free to decide. So I would not characterize this as there is no free will. I would characterise this as what we mean by free will is just a complicated thing happening in us.

I do worry that in series 22, episode 7, we become overly ambitious in hoping that in 28 minutes and 27 seconds, we'll come up with satisfying definitions of what time is and the definition of free will i once tormented a poor student by um we had a we had a discussion about free will he was trying to convince me that there was some sort of yeah extra extra physical free will and i said no you're not you're not free you're not free to pick up that pencil and stab me in the eye. I know for sure that you're not going to do that because your mother raised you right.

And he was obviously, you know, I could see he was really conflicted. He was like, could I? Could I pick up the pencil and stab you in the eye? Just to prove that I have free will. Now, Brian, you've done free will, what is time and quantum gravity? We're going to have to stop now. I love the... That temporal language thing, I think, is great.

There's a lovely... One of the TED Talks you did, Colo, where you actually start off by saying, I have 15 minutes to persuade you that time is an illusion. And I thought, of course, if you manage to do that in 15 minutes, you no longer have 15 minutes to do that you have as long as you want it's kind of an intriguing all those little traps of our language um Mark I wanted to ask you about about I suppose the inspiration sometimes this science and you know when you were writing Doctor Who and uh there's a lovely thing the other day I was doing a virtual pub quiz and someone said could we do a virtual pub quiz on

Doctor Who next week? And the quiz master said, no, we've tried that. They're too pedantic. But I wondered when you are playing around with notions of time, do you does, you know, kind of reading articles and watching lectures about time. Does that inspire narrative sometimes? Or do you think, oh, God, if I get this slightly wrong, I get loads of letters saying you've misunderstood the idea? I think you've got to I mean, you've just got to go with the story, haven't you? As I say, you know, some things like the idea of imprints of or memories of the future that immediately makes your hair stand on anything. That's that's a beautiful idea, isn't it? But Doctor Who particularly can get into a lot of trouble with that because, of course, it's pseudoscience. It's beautiful nonsense, you know,

dimensional transcendentalism, objects fitting inside smaller objects. It's sort of plausible in some sort of distant way. But there have been occasions in Doctor Who's past, famously, when a man called Christopher H. Bidmey took over as script editor for tom baker's last season he tried to to sort of instill some real science into his whole story about entropy and and and all we sort of 14 year old fanboys got very excited because it was proper and serious and correct and of course in the end it's just as nonsensical as all the rest of it uh so i think you've just got to take the rough with the smooth, as they say, and enjoy the stuff. Do you try to be consistent?

No, no, no. Do you try to be consistent a la Bill and Ted? So Bill and Ted, Terminator, they're consistent in the sense that you don't go back and change something that's already happened. That's very satisfying. You're consistent within one story. I think you're consistent within one story because then someone else comes along and flatly contradicts it one week later or several years later that's deaf that definitely happens all the time you know but I think on it's because on on in on the story level

you it has to obey its own rules that week but i mean doctor who begins really with the doctor say you can't change history not one line and then proceeds to change history for the rest of its run so that there aren't really any rules in that sense have you had a favorite there's that lovely phrase i'd love lovely term i can't remember with you first to me antis appointment the excitement that some Doctor Who fans get at the fact they feel they're going to be let down by tonight's episode. It's actually more exciting than being, you know, fulfilled by it. Have there been certain ideas where you think,

I cannot believe that this many people have written to correct this particular idea of science in a time travel show which zooms around in so many different directions? There was a recent, a couple of years ago, there was a Peter Capaldi story called kill the moon where the moon turned out to not be a moon but to actually be an egg which had been sort of left there for millennia and people got so cross about it and you sort of think, well, but why? Because fundamentally the programme is about an alien being who can change his appearance countless times,

who travels time and space in a dimensionally transcendental police box. But an alien moon egg is absolutely beyond the pale. I remember reading the sort of correspondence on that, just thinking, well, I don't understand it. This bit is fine. This bit is absolutely impossible but there we are I want to throw a complete spanner in I know we were at the end of the show right we've recorded for far too long that so I'm going to ask in one minute because we're talking about Doctor Who for Faye and Carlo uh time travel into the past wormholes those sort of things almost yes or no i'll give you one minute each to say whether you

think that time is ordered in that way in the sense that obviously you've described carlo at the start of the show we can go into the future with essentially as fast or as slow as we want relative to someone else into the past yes or no so first of all i have to answer the question which is my favorite doctor who and that's tom baker to answer the question, which is my favorite Doctor Who, and that's Tom Baker. So the question about time travel into the past, if the approach to quantum gravity that I work on, which is called causal set theory, is right, if it's along the fiat, but not quite. So yes, unfortunately, that would not be possible. So yeah, my answer is no. My answer is the following.

It's a bit articulated. If by time travel you just mean that in space-time you can follow the future line and come back to yourself, I don't see reasons for this not being possible. So I think generativity has its solutions and it might be possible. However, if by time travel you mean a person with memories going back to the past and remembering the future, I think this is not possible because memories demand a growing entropy, and you cannot grow along a line all the time.

At some point, when you come back yourself, you have to have the same entropy. So once you have a closed time-like curve, there's a minimum entropy and a maximum entropy, and so the actual experiential direction of time start from somewhere and and join at the maximum so i don't think i can have memories of me older because of entropy because of thermodynamics but you could physically you in some sense it's wonderful isn't it mark that that would be a kind of a curse almost you could actually meet yourself but you'd not time time travel is definitely true because i've i've got a time machine right here for those listening at home it is the most beautiful model from the

george powell uh time machine the one with rod taylor it's a really wonderful thing all i have to do is shrink myself can we do one on shrinking oh yeah we'll tell you what we'll do fantastic voyage next week uh we'll um well thank you very much and uh by the way anyone listening if you have been affected by any of the subjects in today's show i would not be at all surprised to be quite honest um but uh we always ask the audience a question as well and today we asked them if you could control time what would you choose to fast forward through matt kick decided he said i'd skip waiting for my daughter to tell me that what she wants for breakfast then i'd skip actually eating it that would save me 90 minutes every morning

that's someone who's enjoying homeschooling isn't it i like this one from someone called throw the throw the pie the 80s so i'd spend less time in those shell suits oh thank you very much mr ike and i hope you've enjoyed the bit about the uh the moon and the egg um carl stone uh oh yeah again all the time i spent rewinding videos in the 1980s this is one for you this is a this is people think this is a plant because it allows robin to do part of his act. But Dave Parker says all the parts of Brian Blessed films and TV appearances where he's not saying anything.

And then listen on an infinite loop until the end of time. Oh, so it's all the bits where he's just going... Still one of my favourite things, was the noises Brian made when Neil deGrasse Tyson was explaining the Large Hadron Collider and Brian Blessey was just at the side, just with his head down, going, I somebody am... It's a beautiful noise, many of it below consciousness.

What have I got? Ben, the time between my girlfriend saying she's getting... No, no, no, we won't do that. That's a song by Eric Clapton. And Tim... That's the one I read. That's why I didn't want to do that. Yeah, no, you're quite right. I got, unfortunately, three words in. Yeah, yeah. I managed to use my future block universe self

to see the end of that and managed to... Oh, my God, I think I might have broken the very framework of time, Brian. Thank you so much to Carlo Rovelli and Faye Dowker and Mark Gatiss. And it's plasticity. Increasing amounts of research offer hope that as we age, we can still be adept at learning intricate new skills and ideas,

despite the once presumed idea that you can't teach an old dog new tricks. Also, that means there might be some hope of me eventually mastering theoretical physics. No. Oh, OK then. Well, I think I could probably train you to balance a biscuit on your nose. Ah, too late, too late. I did 12 minutes with a Jaffa cake the other day. It was only the wasp out of my nostril that got me.

No, Jaffa cakes aren't biscuits. They're defined as cakes, right, for tax purposes. I mean, in fact, I refer you to United Biscuits UK Limited, number 2, 1991, BBC 818. The judgment was fascinating on Jaffa Cakes, let me tell you. They said, I quote, I come to the conclusion that Jaffa Cakes have characteristics of cakes and also characteristics of biscuits or non-cakes.

I conclude they have sufficient characteristics of cakes to qualify as cakes within the meaning of section 5 group 1 item 1 if it be relevant I also determine that Jaffa Cakes are not biscuits I therefore allow the appeal right so the reason we did that bit was for anyone who's missing in our time at the moment we felt it had

that kind of Melvin Bragg here the Jaffa Cake cake or biscuit AC Grayling anyway thanks very much for listening. Bye-bye. Turned out nice again. Turned out nice again. Hello, I'm Dr Hannah Fry.

And I'm Dr Adam Rutherford. And we present the curious cases of Rutherford and Fry. That's me and her. Certainly do. And every week, what we do, we take a listener question, an everyday mystery, if you will, and we try and investigate it. Using the combined powers of science, books, and and occasionally the internet sometimes we just look it up but anyway we are back with a new series

that's investigating queries like why do our tummies rumble can we make it rain and what exactly is the point of wasps what is the point of wasps it's the end bit the other end of their faces lols i was really pleased with that you can hear all the answers to these questions It's the end bit, the other end of their faces. Lolz. I was really pleased with that. You can hear all the answers to these questions and more by subscribing to The Curious Case of Rutherford and Fry on BBC Sounds. Or wherever you get your podcasts. In our new podcast, Nature Answers, rural stories from a changing planet, we are traveling with you to Uganda and Ghana to meet the people on the front lines of climate change.

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