Transcript:

Brian Keating: [inaudible] aspects of the same thing?

Jim Al-Khalili: They’re being sloppy. In modern physics, according to Einstein, yesterday and tomorrow are equally real as today. There is nowhere in modern physics, our understanding of the laws of physics, that would explain that flow of time. Without gravity, there’s no time. Without time, there’s no gravity.

Brian Keating: You can travel anywhere in space and come right back. But we cannot travel to yesterday. Physics still can’t say why. Jim Al-Khalili is a theoretical physicist at the University of Surrey who admits some of his own ideas are somewhat fluffy. Hawking said asking what came before the Big Bang makes no sense. Yet a growing number of cosmologists disagree, and Jim isn’t sure if either camp is doing physics yet. Near the end, he names a good friend he thinks is very, very wrong about AI.

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## Which is more real, tomorrow or yesterday?

Brian Keating: Which is more real, tomorrow or yesterday?

Jim Al-Khalili: Well, in a sense, neither of them exist anymore. Well, tomorrow doesn’t exist yet. Yesterday has existed and it’s gone. And it’s only the present moment that is real. But I think the common sense view would suggest that yesterday should be more real than tomorrow simply because we have records of yesterday, memory of yesterday. We know it’s happened. Tomorrow has yet to happen. Even in what we call a deterministic universe, where events will unfold in some way that is fated to happen, it’s still something that we can’t predict. So we’d say that yesterday is more real than tomorrow. But in modern physics, according to Einstein, yesterday and tomorrow are equally real as today. This is what’s called the eternalist view of time, which is to say that all times exist. We just happen to live in the present moment, so it seems special to us. But all times are equally real.

Brian Keating: And when we think about the different definitions of time, we have biological time, we have thermodynamic time, we have psychological time. I looked over yesterday where I used to have a toddler and now I’ve got a teenager. The kind of notion that these clocks should be somehow related to each other is to me a great mystery. And yet we physicists, we talk about the fabric of spacetime, Jim, and we wax poetically about space and time are one uniform fabric. Well, I don’t know about you, but if I want to go to St. Louis or Surrey or the South Pole again tomorrow, I could figure out a way to do it. If I want to go to yesterday, I can’t. So are physicists lying when we say things like fabric of spacetime, and space and time are just aspects of the same thing?

Jim Al-Khalili: They’re being sloppy. What I’d say, because Einstein’s lesson when he developed his special theory of relativity in 1905 was, it’s certainly correct that space and time get mixed up. So when you’re looking at different clocks that are moving relative to each other, they tick at different rates. And the way to understand that is to appreciate that we don’t live in a universe that runs according to Newton’s idea of time, which is to say some cosmic clock that ticks by the seconds, minutes, hours, years, the same for everyone regardless of how fast we’re moving relative to each other. Einstein says, “No, time and space get mixed up.” But I think that’s then the mistake that’s made. The fact that time and space get mixed up and you can only really talk about four-dimensional spacetime doesn’t mean that time is just another axis like the axes of space. I’m here on the south coast of the UK. You’re over in California. Where I am in space is real to me. But I also know that where you are in space is real to you. And I know that anywhere on the moon is real, or somewhere on some distant galaxy. And I can go to those places and come back here. So you’re right. Time is very different. I can’t go to some other time in the past or the future. I am constrained. Psychologically, we imagine that we’re drifting along the time axis, that time is flowing, it’s carrying us forward. There is nowhere in modern physics, our understanding of the laws of physics, that would explain that flow of time, that movement through time. That seems to be something that we experience. So I’m not so keen on the different types of time as such. For me, time is time, and events happen and there’s cause and effect and there’s ways of measuring it. There’s ways of determining how fast clocks are ticking, but there should be one underlying thing that we call time. It’s not absolute, but it’s there for any given person. Time is time.

## Time is what a clock measures

Brian Keating: Yeah. And I think that’s the kind of language, the paucity of the economics of language that we have. When I asked Frank Wilczek many years ago on the podcast, “What’s time?” he said, “Time is what a clock measures.” Oh, thank you, Frank. That’s really helpful. But sometimes it does seem like maybe that’s the best that we can do. Frank’s a theorist. I asked it to Bill Phillips, who won the Nobel Prize as well and works at NIST and the accurate measurement of time to the [inaudible] or fractions of a [inaudible] that you so delightfully narrate in this book. What is time? Is it just shut up and look at your watch?

Jim Al-Khalili: In the same way that I’ve never been a fan of the Copenhagen view of quantum mechanics, or certainly the shut up and calculate version of it, I’m also not a fan of this definition that time is that thing that clocks measure. No, we invented clocks as instruments to measure durations, intervals between events. We invented the units that we measure time in, the hours, the minutes, the seconds and all those fractions of seconds, and ultimately they’re based on certain periodic things that humans have discovered, like the rotation of the earth on its axis, the orbits around the sun. But none of that means that time isn’t a real thing that exists independently of us. Time would exist even if there were no clocks that would measure it. It is the thing that we measure with clocks. But I think that does time a huge disservice to suggest that all it is is that thing that we measure, in the same way that it might say space is just that thing that you measure with a tape measure. Space is real and I think time is real too.

Brian Keating: Maybe, as I was reading the book, I thought maybe time can be defined independently, almost the way that we use in differential geometry a vector and a dual, a one-form versus a vector, and that the thing that’s uniting them is change, right? So if you have change, there’s some velocity and a gradient in some space. Is time sort of what we just manifest, but what’s really fundamental, non-emergent, is change? Whatever that means.

Jim Al-Khalili: Is change. Again, I know, philosophers of physics, physicists studying the foundations of physics, and indeed philosophers going all the way back to antiquity have argued about this issue. What is more fundamental? Is it time that’s more fundamental? Is it change? Can you have time without change? Can you have change without time? In my view, of course, change needs time, but I don’t think time needs change. In a universe where nothing was changing, I could still imagine time is ticking by, even if there are no clocks to measure the ticking of time. Very often people talk about the standard example when you talk about the second law of thermodynamics, and you have some isolated system, say a box of gas, and if it starts off in some highly special ordered state, like all the molecules in one corner, and as entropy increases those molecules are basically spreading themselves out until they reach what we call thermal equilibrium, then that doesn’t mean that time isn’t flowing by. There may be no change, or there may be certainly no way of even appreciating the direction that time is moving. That’s tough luck on us, right? Because all you’re seeing is molecules bouncing around randomly. You could take a movie of it and run the movie backwards and you wouldn’t be able to tell. But I still think time is going by even when there’s no change. You look at a stone and it’s not doing anything, just sitting there on the beach. But there is time and there is change, because outside of that there are things that are changing. Okay, you say, right, well, let’s include the surrounding environment of the stone and freeze all of that. Well, fine, there’s still time flowing by outside of that, and you can go out larger and larger and larger layers as you include more of the environment until you reach the whole universe. Then, right, okay, fine, you’re the cosmologist. I would argue that once you reach the whole universe, if you had this ability of having the god’s eye view of sitting outside of spacetime, sure, it may be there’s no change. It may be that it’s timeless. As famously Wheeler and DeWitt gave us this wonderful equation many decades ago that suggests there is no time once you’re describing the entire universe. But we’re stuck inside the universe and there’s always change somewhere, and therefore there is always time.

## Was there a before the Big Bang?

Brian Keating: But I do think of this, the statement by Stephen Hawking that I heard many times, asking what happened before the Big Bang is like… But that’s not really canonical anymore. There are many proposals from people I’ve had on the podcast, ranging from Julian Barbour, mentioned Paul Steinhardt, Neil Turok, many different scientists that throughout the history, from Tolman and others, who speak about time before the Big Bang. The thing that puzzles me, Jim, and I’m seeking my psychological advice, I need to resolve this anxiety, but how can time come into existence? It’s such a strange thing. Space, you could see space embedded in a zero-dimensional point perhaps, and embedded in an infinite-dimensional… at least we have notions of a Hilbert space. But can we think of time? It’s sort of like there’s a reaction, but there’s no action.

Jim Al-Khalili: If time begins at the Big Bang… Stephen Hawking’s original idea many years ago, this no-boundary proposal, suggested that as you get closer and closer to the Big Bang, time as I understand it becomes more like a direction in space. It’s a bit like, for those who study the physics of black holes, when you fall through the event horizon of a black hole and you’re then sort of falling down towards a singularity, because spacetime is so curved round, that radial direction towards a singularity becomes a direction in time, not a direction in space. So hitting the singularity is as inevitable as hitting tomorrow, because you can’t stop going through time. The Big Bang is the opposite of that. Going back, well, according to Hawking anyway, if all you’re sort of thinking about is what general relativity tells us, then you get closer and closer to the Big Bang, time becomes more like a spatial direction, and then it is a bit more like getting closer and closer to the South Pole, and you get to the South Pole and say, keep heading south, and of course every step you take is north again. But that is one picture. That was Hawking’s idea of how time could begin at the Big Bang. It’s not that there was no time and then there is time. It’s more of a gradual time emerging from something that looks more like a spatial dimension. Of course, as you say, asking the question, what was before the Big Bang? That used to be, sort of, physicists would just dismiss those ideas. If you think hard, deeply about it, it’s nothing more than metaphysics, but it’s basically there can be no before the Big Bang, ’cause there was no time to embed the word “before” in. It’s just meaningless. A lot of cosmologists are coming up with neat ideas to suggest that maybe there was a time before the Big Bang. The Big Bang was simply the beginning of our universe. But you know what? What if our universe is a bubble that sort of popped into existence within some higher-dimensional multiverse? What if Neil Turok’s and others’ idea of this mirror universe? The Big Bang may just be a point about which time moves symmetrically in both directions. So time moves forward for us, but there could be another universe before the Big Bang that’s heading in the other time direction. They’re in our past. If there’s any intelligent life in that universe, they would think that we’re in their past. These are neat ideas, and I think it’s fun to be asking questions like, was there a before the Big Bang? The fact of the matter is we do not know. It’s even argued by many that this isn’t really physics, right? Because physics is an empirical science. Until you can come up with some sort of experiment to test your hypothesis, it’s just a neat idea. And I do sometimes find it amusing that, I mean, correct me if I’m wrong, but a lot of papers in cosmology and theoretical cosmology don’t get beyond the preprint stage, because the authors publish a paper, then six months later they’ve changed their minds and they’ve moved on to something else. [laughter] So I don’t know. They’re fun things to think about. I’m not sure if we’re at a point yet. They’re moving from metaphysics into proper physics, but we’re not at a point yet where we can say these are established ideas that we can test.

## In a collapsing universe

Brian Keating: Yeah. As my friend, and probably yours too, Sir Roger Penrose used to say, the safest thing in all of physics was to make a bet with Stephen Hawking, because he’d always change his mind, and so he’d be right as often as he’d be right. But yeah, speaking of Hawking, as you mentioned, in *A Brief History of Time*, which you’d think would address this question, he introduces several things like M-theory and string theory as if they’re proven. And he does motivate the no-boundary hypothesis as if it’s sort of accepted. And even inflation, at that time in 1989, something wasn’t accepted. And he introduces the no-boundary proposal. And he goes, “I want you to stay with me.” I know it’s dangerous, and having equations in the book will reduce its book sales by half, but mentioning God will double the book sales. So it kind of counterbalances itself, and says at the end of the book, the mind of God, if you have string theory. But he says, “Here’s what we’re going to do. We’re going to do this thing called a Wick rotation,” and as you said, it’s going to turn time into space. “But don’t worry, it’s just a simple trick. It’s just a mathematical trick.” And then the whole predicate of the book is based on this trick, which nobody really understands what a Wick rotation is. And you do as good a job as anybody is sort of explaining the problems with that idea. But I do want to ask this question that stumped you as a student, stumps me to this day. What does it mean that in a collapsing universe, you wouldn’t necessarily have a different perspective on the future versus the past? You wouldn’t be able to distinguish, if we’re in a collapsing universe, it wouldn’t be like things run back in time. Can you explain that? Unpack that for me.

Jim Al-Khalili: Thomas [inaudible] came up with this idea that the Gold universe would suggest that the Big Bang is this very beginning of time. The universe was in a very special state. It was like a clock that’s wound up, ready to unwind. Then the universe expands, gets bigger, but it reaches some maximum expansion, then recollapses in on itself back to a big crunch. But the idea there was that there’s a symmetry. The big crunch should be just like the Big Bang, which is to say a highly ordered state, a low entropy state. Now, if the second law of thermodynamics says as the universe expands entropy is increasing, and there are lots of different measures of entropy, but let’s just say the universe is becoming more disordered, becoming a less special state until it reaches some maximum expansion. That’s the maximum entropy to get to. Then it starts collapsing. Now it’s going to get back to something like the Big Bang. Entropy has to decrease in the collapsing phase. And so the idea there is that our psychological perception of the direction of time should follow the thermodynamic arrow of time. Thermodynamics should operate in the brain as they would in any other part of the universe. And so if this entropy is decreasing in the collapsing phase, that means the thermodynamic arrow of time has switched over. It’s now pointing towards the Big Bang, which means our psychological arrow is pointing towards the Big Bang. So now the Big Bang is in our future. He published a paper in the mid-’80s, and this is the paper I’ll talk about in the book, and as a grad student I read it and didn’t agree with it, and I had lots of ideas about why it was wrong, but I didn’t have the maths background to get beyond the first or second page of the paper. So I did nothing about it. But it’s this idea: what would happen if you’re in a rocket and the universe is just about to reach the point of maximum expansion at 3:00 p.m. one afternoon? What happens at one minute to three? You say, “Oh, the 3:00 is in my future, and the universe is still expanding.” You get to 3:00. One minute past three, the universe is collapsing. But now I’m still thinking that maximum expansion point is in my future, and it just didn’t make any sense to me. It’s a really confusing idea, the fact that arrows of time should flip over in the Gold universe. Hawking himself, of course, to his credit, in *A Brief History of Time* owns up. This is another example of him admitting that he’d made a mistake, and he said, no, this nonsense what I said in that paper. And I remember reading his book, *History of Time*, on a train and grinning broadly, with, yeah, I thought that, damn it, why didn’t I write something up at the time?

Brian Keating: Why didn’t I use the Wick rotation?

Jim Al-Khalili: [inaudible] Exactly. If I knew what it was at the time. [laughter]

## What did Einstein believe about time?

Brian Keating: You write very beautifully in the book that nothing more compellingly shows just how conceptually different physical time and manifest time is than the consequences of Einstein’s theory of general relativity. What did Einstein believe about time? What did he say and what did he believe? Because it seems to me, just as his ideas about religion and God, it’s very ambiguous and it’s not clear exactly what he believed. But you’re a scholar. What did he truly believe?

Jim Al-Khalili: I think, I mean, it took him a while to really appreciate what his mathematics was saying, particularly after his special theory of relativity. It wasn’t Einstein who said time is the fourth dimension, time and space have to be combined together. That was Hermann Minkowski. But of course Einstein picked up on that idea and led to his notion of a block universe, that space and all times coexist. This is the eternalist picture. I think he genuinely believed that all times were equally real. Our now, our present moment in our reference frame, is some point along the time axis in our reference frame, and that point we perceive, psychological time, manifest time, to be creeping along the time axis. For Einstein, there was nothing special about that. There’s an example I’ve started using. If I’m reading a book, say a novel, and I’m on page 150, then all the pages before 150 I’ve read. I know about them, I remember them, they’re in my past. All the pages after 150 are in my future. I have yet to read them. But if I close the book, page 150 is no longer special. All pages are equally real. And I think that Einstein generally believed this notion that all times are equally real. There’s nothing special about the present moment. There is the story that he was consoling the widow of a good friend of his at the funeral by trying to persuade her that she may be grieving now because of the loss of her husband, but all those moments in the past when he was alive are just as real as now. He’s just as alive in spacetime as he’s ever been.

## Loschmidt’s paradox

Brian Keating: There’s a character in this book that makes an appearance. A lot of paradoxes when it comes to time, but none more kind of grievous to the mind when we encounter it for the first time. I never encountered it before I read this book on time. It’s called Loschmidt’s paradox. Sounds like Locksmith, which probably is, or maybe it’s a lake or something up there in Scotland, you’ll tell me. And it seemed to evoke some notions of Maxwell’s demon, but no demons are harmed in the making of this paradox. So first, can you explain Loschmidt’s paradox? What is it? Why is it so significant that you devote so much attention to it?

Jim Al-Khalili: Loschmidt’s paradox was something that was known and discussed and debated in the 19th century, the mid-19th century, by people like James Clerk Maxwell and Ludwig Boltzmann. These are guys that were developing statistical mechanics and thermodynamics. And it’s the idea that there’s this real puzzle between how time is perceived in two different areas of physics. On the one hand, you’ve got the equations that describe how systems change over time, the dynamical equations of physics. Now, these could be Newton’s equations of motion. It could be Schrödinger’s equation of quantum mechanics. It could be the diffusion equation, the electromagnetic wave equation. All of those equations have time coming in as a parameter, right? The lowercase t. And you solve those equations to get a solution, and then you plug in a value for that t, and that tells you what the thing that you’re studying is doing at that moment. These are equations that describe the fundamental ways that systems change over time in all areas of physics. And what’s special about them is that you can run them forwards and backwards in time without breaking any laws of physics. Sometimes running them forward and backwards, it’s quite symmetrical. So this is what the system is like. Now if I evolve the equation forward 10 seconds, the system will have changed. It’d be something else. If I evolve it back in time 10 seconds, it’ll look exactly like it does 10 seconds in the future. Sometimes it doesn’t look the same, like the universe expanding. 10 seconds from now it’ll be bigger, but 10 seconds ago it was smaller. But that doesn’t mean that you’re breaking your laws of physics. If you turn time in the opposite direction, you just describe a collapsing universe rather than an expanding universe, and so on. So these equations are all said to be time reversal invariance, in essence symmetric in time. And yet Loschmidt’s paradox says, hang on a minute, the second law of thermodynamics says time is irreversible. It only moves in one direction. I get older, balls roll down hills, ice melts in water, the cream mixes in coffee. How do you get that irreversibility from fundamental equations that are reversible, that are symmetric in time? This is the paradox, the problem of physics that people worried about.

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## The block universe and the past hypothesis

Brian Keating: The other character, another major topic in the book, involves things like the block universe and the past hypothesis. I’ve had on David Albert, and I kind of feel like they’re both sort of ad hoc axioms that are bolted on to a framework to make certain people feel more comfortable about their choice of paradigm. I used to think, why do we need an interpretation of quantum mechanics? I don’t have an interpretation of classical mechanics. But it seems like we do need an interpretation of thermodynamics. But what is the past hypothesis and the block universe? How are they related? What, if anything, do they do besides kind of maybe assuage people’s feelings at 3 in the morning?

Jim Al-Khalili: I sort of think of them as differently. You have these different areas of physics. You have the dynamical equations of motion, in which time appears as a parameter. That’s what’s called coordinate time, right? Then you have the block universe and spacetime, where time isn’t a coordinate. It’s an entire axis. It’s the fourth dimension of spacetime, not the same as the dimensions, the axes of space, but nevertheless it’s part of four-dimensional spacetime. Then you have a third picture, which is thermodynamics, which says time isn’t a parameter, it’s not a dimension, it’s an arrow. It’s a direction. It points in one direction only. That is linked to this Loschmidt’s paradox. How do you get an arrow of time when the fundamental equations of physics are symmetric? The past hypothesis is a way of finessing that. It’s almost cheating, because what it says: imagine today the universe has a certain value of entropy. This is the measure of disorder that the second law of thermodynamics says is always increasing. Tomorrow the entropy of the universe will be bigger. Yesterday the entropy of the universe should be smaller. But if time is symmetric, then yesterday the entropy should increase just like it increases tomorrow. But what’s special about today? I could have measured the entropy yesterday. They’d have told you, no, it was definitely less than it is today. So what if you push the special moment back to last week? Now yesterday had lower entropy than today. Today has lower entropy than tomorrow, and so on. But what was special about last week? So the past hypothesis is the idea that you push that special moment all the way back to the Big Bang, after which, at all times, entropy is increasing. You say, yeah, but surely it’s symmetric. What about entropy increasing to the past? Ah, but there’s no past before the Big Bang. So we’ve finessed the problem. We don’t have to worry about entropy increasing to the past, because we start at a low entropy. And that’s what this past hypothesis is essentially saying. It solves this problem of time reversal invariance. What it breaks is time translation invariance. It says, is there something special about a particular moment? It’s an add-on, right? To help us understand why time appears all around us to be pointing from the past to the future and doesn’t run in both directions at once.

## The past hypothesis for free

Brian Keating: And you connect that to fine-tuning. Can you explain that argument and where it succeeds or fails?

Jim Al-Khalili: We seem to need two things to describe the Big Bang. One is the past hypothesis, that the universe started off in a very special state, low entropy state, highly ordered state. And the other is that fundamental constants of nature, the forces, the speed of light and all the fundamental constants, had to be a certain value in order for the universe to evolve and be the way it is today. If anything was, the charge on the electron was a bit greater, atoms would be unstable. If gravity was too weak, stars and galaxies would never form, and so on. Everything had to be fine-tuned just right to give us this interesting, complex, structured universe we have today. My argument is that I don’t know why those initial conditions were the way they were. I don’t have an explanation for the fine-tuning, but my argument is that fine-tuning itself gives us a highly ordered, low entropy universe. It gives us the past hypothesis for free. So I don’t see them as two separate things. We need fine-tuning and a past hypothesis? I would argue we need fine-tuning, and the past hypothesis emerges from that naturally.

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## Without gravity, there’s no time

Brian Keating: The last major topic before we wrap up, I know it’s late there, is this new proposal that you have, which kind of is reminiscent, at least rhymes with Einstein’s conception of time and gravity being uniquely intertwined. And you give it a new spin, literally a new spin, in a quantum mechanical version of time’s reality. You call it not emergent, but in a real, true, honest-to-goodness physicalist sense. First of all, can you explain, again going back to our friend Einstein, what was the intimacy between gravity and time? Do we have time if there’s no gravity, in a Minkowski flat universe? And then, if you could explain your quantum proposal as well.

Jim Al-Khalili: So Einstein, he wrote a famous book during the First World War, his theories of special and general relativity, essentially a popular science book about relativity theory, and it underwent many editions. And when a publisher tells an author, “We’re going to have a new edition, do you have any changes?” they might want to tweak it or add a few things. Einstein didn’t tweak the main text, but he would add appendices to the book. And the very final appendix he added was just a year or two before he died in the 1950s. And it’s there where he very beautifully outlines what the connection he believes is between gravity and time and space. He says that unlike the other fields, force fields, say in quantum field theory, which sit on spacetime, you need spacetime, the canvas in which reality sort of plays out, unlike those, the gravitational field is the spacetime. It’s a manifestation of spacetime. So if there’s no gravitational field, Einstein would argue there is no spacetime. He’s gone beyond Minkowski spacetime. The Minkowski spacetime is, there’s no gravity, right? It’s not just flat spacetime, because in the limit that gravity gets weaker and weaker, Einstein would say if there’s no gravitational field, you switch it off, then you’re removing spacetime itself. And if you have spacetime, then you have a gravitational field. So I think it’s intimately connected. And if spacetime is space and time, then without gravity there’s no time, without time there’s no gravity. So gravity isn’t just this extra fourth force of nature that we’re trying to unify with the other quantum forces, electromagnetism and the nuclear forces. It’s something deeper, something more fundamental. And for me, I think this is the real challenge and reason why we haven’t found a theory of quantum gravity yet, because we’re still trying to fit round pegs in square holes by trying to combine gravity in the same way that we dealt with the other forces. Gravity, the gravitational field, is spacetime.

## The entanglement past hypothesis

Jim Al-Khalili: I came to it because I was trying to understand, is there another way of coming up with the past hypothesis, or in what sense was the universe special? I was led to it from work I did with actually a colleague of yours in UC San Diego, Eddy Chen, who’s a philosopher of physics, and we published a paper together a couple of years ago on what we call the entanglement past hypothesis. So rather than saying the universe was in some highly ordered thermodynamic state, a low entropy state, we had to come up with a way of defining this notion of entropy, this measure of disorder, down at the quantum level. So some form of quantum entropy. We weren’t the first to do this. Lots of people have views on what this quantum entropy should be. Von Neumann entropy, it’s often called. But our suggestion was that maybe at the Big Bang quantum entanglement was very small, that the universe started off in a very pure quantum state rather than a special thermodynamic state, and gradually everything gets entangled with everything else, and as entanglement increases, that gives us an arrow of time pointing from the Big Bang. So rather than having a thermodynamic arrow, which says thermodynamic or Boltzmann entropy is increasing, we’re saying entanglement is increasing. Entanglement entropy is increasing, starting from a minimum. A lot of people are interested in this idea of whether time itself is emergent from the quantum realm, whether space is emergent from the quantum realm, and is quantum entanglement the fundamental thing that defines how all the other attributes of our universe come about from. It’s very fluffy. It’s very hand-wavy. And I’m relatively new to the game of working in foundations of physics, so I’m sort of having fun with this, and I keep discovering that if I have a neat idea, I find out this is actually really well known by people in the field. That’s fine. I don’t mind.

Brian Keating: Well, thank God you’re officially retired, or emeritus now, ’cause…

Jim Al-Khalili: [inaudible] I don’t have to worry about my CV or publication.

Brian Keating: Your tenure case would be perhaps a little bit compromised.

## Judge your book by its cover

Brian Keating: Okay, I’ve got three relatively rapid-fire questions. The first involves what you’re never supposed to do, but I love to do, which is to judge your book by its cover. We know we’re not supposed to do it, but Into the Impossible, there’s nothing to it. Let’s take a look and judge some books. So take us through the title, the subtitle, and the cover art, please.

Jim Al-Khalili: [inaudible] All of this credit goes to the team at Princeton University Press who published the book. When I was writing it, I had all sorts of ideas for titles: *Adventures Through Time*, *The Reality of Time*, *The Story of Time*, and my publishers said, “Look, Jim, leave this to us, right? We’ll come up with something.” I have to say the title, *On Time*, is brilliant. I absolutely love it. As soon as I saw it, I thought, “Oh, yeah, that’s it.” Of course, the subtitle, *The Physics That Makes the Universe Tick*, then tells you what it’s all about. But it lends itself to humorous comments about, when I’m giving talks on the book, I make sure I turn up on time. It’s literally the title of my book. I better not be late. The cover design, I think, is also very nice. They did talk me through the details of what it all means. What they didn’t want was to have some sort of clock, even a Dalí sort of melting clock falling into a black hole. No, just have all these lines pointing towards the middle. There’s a rationale for it, and I wish I had it off the top of my head to explain what they meant by it, but I think it’s very striking.

Brian Keating: But I think there’s an Easter egg here. The orange is a *Clockwork Orange* reference. Come on, Jim. It’s got to be a Clockwork [inaudible].

Jim Al-Khalili: Hang on now. [laughter]

## Do they experience time?

Brian Keating: Okay, two more questions. The required, mandated in the state of California, talk about AI. But a lot of times I hear about artificial intelligence in this concept of, it’s going to do everything that humans can do. And I point back to the topic that you mentioned in the book. I call it Einstein’s elevator: an object in free fall. As he said, “My happiest thought was that an object or observer in free fall experiences no gravitational field,” which would seem to indicate they obtain no time passage either, according to Einstein, if you can nullify it in the Einstein equivalence principle. By the way, I use that as a counterproof that AI can’t do physics the way that Einstein could, because it can’t have a happy thought and it can’t feel free fall. But maybe they’ll get around that. But my question is, can they experience time? Do they experience time? They don’t have psychologies. They have some ways of representing memory, but often it’s quite bad. Can artificial consciousness participate in the entanglement that you maybe potentially are beginning to subscribe to?

Jim Al-Khalili: I don’t see why not, actually. I think you’re right, and artificial intelligence just being intelligent and being able to solve problems that humans can’t, even coming up with some notion of creativity, that’s one thing. But being self-aware, I think experiencing time, psychological time, manifest time, is connected somehow with this notion of being aware of the sense of self. When we think of the past, what we’re doing is accessing memories in our brains, where we’ve stored as a record of events that have happened in the past. But I think the notion of experiencing time moving, anticipation of the future, remembering the past, being aware that we live in the present moment, I think that requires a sense of knowing oneself, the introspection, sentience. Artificial intelligence is a long way from that. However smart LLMs are, and despite the likes of my good friend Richard Dawkins saying that he believes that Claude is truly conscious, I think he’s very, very wrong. I don’t think that means that AI wouldn’t one day become sentient and self-aware. I think that may not be thinking like a human being will think. It may not feel joy and anger and envy or whatever in the same way we do, but it might become self-aware.

## Has time taken on a different meaning?

Brian Keating: Very good, Jim. Well, last question. You became a grandfather, I think, since we last spoke. Is that correct? I think. Has time taken on a different meaning since that happened? Is it qualitatively different since becoming a father? They say that grandfathers and grandchildren get along because they have a common enemy. How does time’s poignancy affect you now as a grandfather, versus as a father, versus as a single man?

Jim Al-Khalili: I think in lots of ways, and also just getting older. I remember when I turned 40, that was great. Well, that’s no problem with that. I turned 50, I had a big fabulous party. I turned 60 and you think, my god, this is getting serious now. So there’s that notion of, I’ve lived more life than I have left. Your sense of one’s mortality, and that’s compounded by the fact that my two children in their 30s. I have one-year-old grandchildren, twins, who are lovely. My father’s still alive. He’s 95 and still in very good health. So I still don’t feel as though I’m next off the cliff edge, as it were. I still have another generation beyond me who’s still alive and well. And I think maybe that would change again when my father passes away and I feel I’m next in line. But you’re right, having grandchildren does give one a sense of the finiteness of time. Changing my grandchildren’s nappies reminds me of changing my children’s nappies, and I can’t believe that that was more than three decades ago. So that notion that time is speeding up and time is running away from me is certainly a very feeling that I’m not unique in experiencing.

Brian Keating: I can’t wait for you to do many more diaper changes, nappy changes, but also many more books, and hopefully we’ll get to meet up someday.

Jim Al-Khalili: I’ll do both.

Brian Keating: Thank you, Jim. Congratulations on the success of this new book. Bye-bye.

Jim just told us that without gravity there may be no time at all. That changes how you think about tomorrow. Subscribe and turn on notifications, and let me know in the comments whether yesterday is as real as tomorrow. And to hear what might have come before the Big Bang, watch my conversation with Neil Turok, linked right [inaudible]