Jim Al-Khalili: Physicists Are Being Sloppy About Time

Physicists say space and time are the same thing. They are being sloppy. Jim Al-Khalili has spent his career on the foundations of physics, and his new book makes the case that the past and the future are equally real, and the laws of physics cannot explain why you can only move in one direction. That is the central mystery of time travel. Subscribe if you want science with evidence, not speculation. The block universe says all times coexist and the present moment is not special. Loschmidt’s paradox says the fundamental equations of physics are reversible, and yet ice never unmelts, cream never separates from coffee. Nobody has solved that contradiction. The entanglement past hypothesis, his own new proposal developed with a colleague at UC San Diego, proposes that the answer is not in thermodynamics but in quantum entanglement growing from almost nothing at the Big Bang. The conversation covers what the block universe actually implies about the future and free will, what Einstein actually believed about whether the past and future are equally real, why removing gravity means removing time itself, and whether AI can ever experience time the way conscious beings do. Al-Khalili says Richard Dawkins told him Claude is truly conscious. Al-Khalili thinks he is very wrong. What you’ll hear: Why Al-Khalili disagreed with the Thomas Gold paper as a grad student but did not have the maths to prove it What fine-tuning of the fundamental constants has to do with why the past hypothesis may not be needed Why the Clockwork Orange Easter egg ended up on the cover of a physics book published by Princeton Why Richard Dawkins thinks Claude is conscious and why Al-Khalili thinks he is very wrong “There is nowhere in modern physics, our understanding of the laws of physics, that would explain that flow of time.” — Jim Al-Khalili CHAPTERS 00:00 Physicists are being sloppy about time 01:14 Which is more real: yesterday or tomorrow? 21:00 Irreversibility from reversible equations 26:32 The past hypothesis: almost cheating 30:52 No gravity. No time. 31:54 The entanglement past hypothesis 35:06 Dawkins thinks Claude is conscious. Al-Khalili disagrees. Get the transcript, fascinating bonus content, and my Monday M. A. G. I. C. Message: https://briankeating.com/yt Enter the debate: https://briankeating.com/ai Have a .edu email and live in the USA? You automatically win a meteorite: https://BrianKeating.com/edu Subscribe: https://www.youtube.com/DrBrianKeating?sub_confirmation=1 Support Into the Impossible on Patreon, get my weekly M. A. G. I. C. Message, unfiltered bonus content, and live monthly Office Hours with me: https://www.patreon.com/drbriankeating Join this channel for perks, monthly Office Hours, and your name in the Member Roster at the end of every episode: https://www.youtube.com/channel/UCmXH_moPhfkqCk6S3b9RWuw/join Featured Guest: Jim Al-Khalili website: https://www.jimal-khalili.com/ On Time (book): https://press.princeton.edu/books/hardcover/9780691267036/on-time Jim Al-Khalili on Twitter/X: https://x.com/jimalkhalili My books: Losing the Nobel Prize (memoir): http://amzn.to/2sa5UpA Think Like a Nobel Prize Winner: https://urlgeni.us/amzn/TLANPW Focus Like a Nobel Prize Winner: https://a.co/d/hi50U9U Galileo’s Dialogue (first-ever audiobook): https://a.co/d/iZPi9Un Twitter/X: https://x.com/BrianKeating Substack: https://briankeating.substack.com Blog: https://briankeating.com/blog Audio-only: https://briankeating.com/podcast #JimAlKhalili #timetravel #entropy #physics #quantumentanglement #briankeating #intotheimpossible Learn more about your ad choices. Visit megaphone.fm/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

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• 0:31Speaker 01

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• 0:59Brian Keating?

  Are physicists lying when we say things like fabric of space-time and space and time are just aspects of the same thing?

• 1:05Speaker 03

  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.

• 1:22Brian Keating?

  You can travel anywhere in space and come right back. But you cannot travel to yesterday and 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. Hey, I just found out that many of you got unsubscribed recently in a YouTube update. Make sure that didn't happen. Please make sure that you're subscribed and don't forget to leave a comment down below. Thanks a lot, it really helps out. Which is more real? Tomorrow or yesterday?

• 2:03Speaker 03

  Well, in a sense, neither of them exist anymore. Well, the 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, a 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, is 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.

• 2:43Speaker 03

  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.

• 2:55Brian Keating?

  And when we think about the different definitions of time, we have biological time, we have thermodynamic time, we have psychological time. You know, I looked over yesterday where I used to have a toddler and now I've got a teenager. The, 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, the fabric of space-time, Jim, and we wax poetically about, you know, space and time are one uniform fabric. Well, I don't know about you, but you know, 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 space-time and space and time

• 3:39Brian Keating?

  are just aspects of the same thing?

• 3:41Speaker 03

  They're being sloppy, is what I would say. Because Einstein's lesson, when he developed his special theory of relativity in 1905, was certainly, 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, 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.

• 4:24Speaker 03

  But I think that's, 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 space-time, doesn't mean that time is just another axis like the axes of space. You know, 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.

• 5:08Speaker 03

  Psychologically, we imagine that we're drifting along the time axis, that time is flowing, it's carrying us forward. There is nowhere in, 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, we, we experience. So I wouldn't, you know, 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 that's, that's, it's not

• 5:52Speaker 03

  absolute, but it's, it's there for any given person, time is time.

• 5:58Brian 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 you know, sometimes it does seem like maybe that's the best that we can do. You know, Frank's a theorist. I asked it to Bill Phillips, who won the Nobel Prize as well and works at NIST and that accurate measurement of time to the yada second or, you know, fractions of a yada second that you so delightfully narrate in this book. What, what is time? I mean, is it, is it just shut up and, and, and look at your watch?

• 6:38Speaker 03

  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, I mean, we invented clocks as instruments to measure durations, intervals between events. We invented, invented the units that we, we, we measure time in, the hours, the minutes, the seconds, and all those fractions of seconds. Ultimately, they're based on certain periodic things that humans have discovered, like this, the rotation of the earth on its axis, the orbits around the sun.

• 7:17Speaker 03

  None of that means that time isn't a real thing that exists independently of us. You know, 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, we, we, 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.

• 7:45Brian Keating?

  Maybe as I was reading the book, I thought maybe time, you know, sort of 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, is time sort of what we just manifest? But, but what's really, you know, fundamental, not emergent, is, is change, whatever that means.

• 8:18Speaker 03

  Is change. Again, I, I know, I mean, philosophers of physics and physics, physicists studying the foundations of physics, and indeed philosophers going all the way back to antiquity, have argued about this, this issue. What is more fundamental? Is it time that's more fundamental? Is it change? You know, 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 were no clocks to measure the ticking of time. Very often, people talk about, you know, the, the standard example, when you talk

• 9:03Speaker 03

  about the second law of thermodynamics and you have some isolated systems, 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 they, entropy increases, those molecules are basically spreading themselves out until they reach what we call thermal equilibrium.

• 9:22Speaker 03

  Then that doesn't mean that time isn't flowing by, you know, 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? You know, because all you're seeing is molecules bouncing around randomly. You could take a movie of it and run the movie backwards and it, 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 that the surrounding environment of the stone and and freeze all of that. Well, fine, there's still time flowing

• 10:06Speaker 03

  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, I would argue that once you reach the whole universe, if you could have 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, you know, as famously Wheeler and Bewick 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.

• 10:47Brian Keating?

  But I do think of this, you know, the statement by Stephen Hawking that I heard many times, asking what happened before the Big Bang is like, but that's not really, you know, canonical anymore. There are many proposals from people I've had on the podcast, ranging from Julian Barbour, you mentioned, Paul Steinhardt, Neil Turok, many different scientists that throughout the history from Tullman and others who speak about time before the Big Bang. The thing that puzzles me, Jim, and I'm seeking my psychological advice, you know, I need to resolve this anxiety, but how can time come into existence? You know, it's so, it's such a strange thing. Space, you could see, you know, space embedded in, you know, zero dimensional point, perhaps embedded in an infinite dimensional, you know,

• 11:29Brian Keating?

  at least we have notions of a Hilbert space. But can we think of time, you know, it's sort of like there's a reaction, but there's no action if time begins at the Big Bang.

• 11:39Speaker 03

  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 when, for those who study the physics of black holes, you know, when you fall through the event horizon of a black hole and and you're then sort of falling down towards a singularity, because spacetime is so, so curved round, that radial direction towards the 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. I think

• 12:22Speaker 03

  going back, well, according to Hawking, anyway, you know, if, if all you're, 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, of how time could begin at the Big Bang. It's not that it's, it's, 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

• 13:06Speaker 03

  of physicists would just dismiss those ideas. So, you know, if, if you're thinking hard and deeply about it, it's nothing more than metaphysics, but it's basically, there can be no before the Big Bang because 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 if our universe is a bubble that sort of popped into existence within some higher dimensional multiverse?

• 13:40Speaker 00

  This episode is brought to you by Indeed. You're ready to level up your business, but the pressure to find the right person to make that happen is intense. Indeed sponsored jobs can help carry the load. It boosts your job post to help it reach quality talent fast. Cut through the chaos with Indeed sponsored jobs. Get a $75 sponsored job credit at indeed.com/podcast. Terms and conditions apply. Hiring now? Then this is a job for Indeed sponsored jobs.

• 14:11Speaker 03

  What if, you know, Neil Turok's and others' idea of this mirror universe, the Big Bang maybe just be a point about which time moves symmetrically in both directions. The time moves forward for us, but there, 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, we, they would think that we're in their past. These are neat ideas. And I think, I think it's fun to be, to be asking questions like, was there a before the Big Bang? That 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,

• 14:55Speaker 03

  it's just a, you know, 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, theoretical cosmology, don't get beyond the preprint stage, because the authors, you know, they, they, they publish a paper, then six months later, they've changed their minds and they've moved on to something else. So I don't know. They're fun things to, 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.

• 15:31Brian Keating?

  Yeah, as my friend and probably yours too, Sir Roger Penrose, used to say that the safest thing in all of physics was to bet, make a bet with Stephen Hawking, you know, because he'd always change his mind. And so he'd be right as often as, as he, as he'd be right. But yeah, speaking of Hawking, as you mentioned in A Brief History of Time, which he 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, you know, at that time, 1989, something was 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 sale. So it kind of, you know, counterbalances

• 16:15Brian Keating?

  itself and says at the end of the book, you know, 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 WIC 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 that, this trick, which nobody, you know, really understands what a WIC rotation is. And you do as good a job as anybody is sort of explaining the problems of that idea. But I do want to ask this question that, you know, stumped you as a student, stumps me to this day. You know, what does it mean that you, you could, in a collapsing universe, you wouldn't necessarily have a different perspective on the future versus the past? Like you, 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?

• 16:59Speaker 03

  Thomas Scott came up with this idea that the gold universe would suggest that, you know, the Big Bang is this very beginning of time. It's a very, 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 re collapses 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

• 17:40Speaker 03

  just say it's the, 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, if 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 operated the brain as, 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. Now it's now pointing towards the Big Bang, which

• 18:24Speaker 03

  means our psychological arrow is pointing towards the Big Bang. So now the Big Bang is in our future. And he published a paper in the mid eighties, 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 of what about why it was wrong. But I, I didn't have the maths background to get beyond like the first or second page of the paper. So, so I did nothing about it. But it's this idea that what would happen if you're in a rocket and the universe is just about to reach the point of maximum expansion at 3pm one afternoon? What happens at one minute to three? You say, oh, the three o'clock is, is in my

• 19:06Speaker 03

  future. And, you know, the universe is still expanding. You get to three o'clock, one minute past three, the universe is collapsing, but now I, I, I'm, I'm still thinking that maximum expansion point is in my future. And it just didn't, it didn't make any sense to me. It's a really confusing idea, the fact that arrows of time should, should flip over in the gold universe. Hawking himself, of course, to his credit, in The Brief History of Time owns up. This is, this is another example of him admitting that he'd made a mistake. And he said, no, I, this nonsense, what I said in that paper. And I remember reading the paper, reading his book, Brief History of Time on a train and grinning broadly

• 19:49Speaker 03

  with, yeah, I thought that. Damn it. Why didn't I write something up at the time?

• 19:56Brian Keating?

  Why didn't I use the WIC rotation, you know, and then my, and

• 19:58Speaker 03

  Why didn't I use the WIC? Commutedly, exactly. If I'd knew what it was at the time.

• 20:03Brian Keating?

  You write very beautifully in the book that, yeah, 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, 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?

• 20:30Speaker 03

  I think, I mean, it took him a while to really appreciate what his mathematics were saying, particularly Artis' special theory of relativity, you know, 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 this, his notion of a block universe that space and all times coexist. This is eternalist picture. I think he genuinely believed that, you know, all times were equally real. Our now, our present moment, you know, in our, in a, in our reference frame is some point along the time axis in, in our reference frame.

• 21:12Speaker 03

  And that point, we perceive it, psychological time, manifest time, to be creeping along the time axis. For Einstein, that was nothing, nothing special about that. There's an example I've, I've started thinking, I've started using. If I'm reading a book, say a novel, and, and I'm on a page 150, then all the pages before 150 are, are, 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 that this notion that all times are

• 21:55Speaker 03

  equally real, there's nothing special about the present moment. There is the story that he tried to, 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.

• 22:21Brian 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 for the first time. I never encountered it before I read this book on time. It's called Lachschmidt's Paradox. Sounds like locksmith, which probably is, or maybe it's a lake or something out 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 the Loschmidt's paradox? What is it? Why is it so significant that, that you devote so much attention to it?

• 22:54Speaker 03

  Loschmidt's paradox was something that was known and, and discussed and debated in the 19th century, the mid 19th century by people like James Clark Maxwell and Ludwig Boltzmann. These are guys that were developing statistical mechanics and, 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 Schrodinger's equation of quantum mechanics, it could be the diffusion equation, the electromagnetic wave equation. All of those

• 23:36Speaker 03

  equations have time coming in as a parameter, right? The lowercase t. And, and you fold 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 they, 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 would have changed. It'd be, it'd be something else.

• 24:16Speaker 03

  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, you know, like the universe expanding 10 seconds from now it can be bigger, but 10 seconds ago it was smaller. But that doesn't mean that you're breaking your laws of physics. You know, 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,

• 24:56Speaker 03

  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, that, you know, the, the, the problem of physics that people worried about.

• 25:13Brian Keating?

  Hey, do me a favor. I know you're enjoying this on YouTube, but make sure you follow me on Spotify too, and leave a review. That helps out more than I can tell you. The other character, another major's topic in the book involves things like the block universe and the past hypothesis. I, I've had on David Albert and, and, you know, I, 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, 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, the past hypothesis in the block universe? How are they related? What, if anything, do they do besides kind of maybe assuage people's feelings at three in the morning?

• 25:56Speaker 03

  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, it's, it's an entire axis. It's the fourth dimension of spacetime. Not, 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.

• 26:31Speaker 00

  This episode is brought to you by Indeed. You're ready to level up your business, but the pressure to find the right person to make that happen is intense. Indeed sponsored jobs can help carry the load. It boosts your job post to help it reach quality talent fast. Cut through the chaos with Indeed sponsored jobs. Get a $75 sponsored job credit at indeed.com/podcast. Terms and conditions apply. Hiring now? Then this is a job for Indeed sponsored jobs.

• 27:01Speaker 03

  It points in one direction only. That is linked to this Lachschmidt'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 what the second law of thermodynamics says is always increasing. Tomorrow, entropy of the universe will be bigger. Yesterday, the entropy of the universe should be smaller. But if, if, if time is symmetric, then yesterday the entropy should increase, just like it increases tomorrow. But what's, why, why, what's special about today? I could have

• 27:43Speaker 03

  measured entropy yesterday. I told you, no, 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 all times entropy is increasing. You say, yeah, but, 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

• 28:25Speaker 03

  this past hypothesis is essentially saying. It solves this problem of time reversal invariance. What it breaks is time translation invariance. It's, is there something special about a particular moment? It's an add-on, right? To, to, to, to help us understand why time appears all around us to be pointing from the past to the future and doesn't run in, in, in both directions at once.

• 28:50Brian Keating?

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

• 28:56Speaker 03

  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, 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, if the charge on the electron was a bit greater, atoms would be unstable. If gravity was, 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.

• 29:37Speaker 03

  My argument is that I don't know why the those that 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.

• 30:07Brian Keating?

  Just a quick interruption from today's sponsor, me, Brian Keating. Since it's Nobel Prize season, I can't resist advertising my three books. First book I wrote is called Losing the Nobel Prize. It's a cosmology story mixed with a memoir of what it feels like to do science at the highest level and spoiler alert, fail. But my next book, Think Like a Nobel Prize Winner, tells the story of nine Nobel laureates who will reveal their insights and actions, how to take your science, or just your job in general, to the next level. It's full of insights and tips that I never would have thought of. And lastly, my book published last year, Focus Like a Nobel Prize Winner, is written for students of all ages to learn the tools and the tricks to compete with yourself,

• 30:52Brian Keating?

  which sometimes can be the hardest competitor of all. Get all three of them, Amazon affiliate links in the description below. And if you've read them already, I'd love it if you leave a review. Every single review helps. Now, back to the episode. 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

• 31:35Brian Keating?

  time? We have time if there's no gravity, and Minkowski flag. And then, and then if you could explain your quantum proposal as well.

• 31:42Speaker 03

  So Einstein, he wrote a famous book during the First World War, his theories of special and general relativity, and it's essentially a popular science book about 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 like 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 and time and space. He says that unlike the other fields, fourth fields, and say in quantum field theory, which sit

• 32:27Speaker 03

  on spacetime, you know, you need spacetime, the canvas in which reality sort of plays out. He said, unlike those, the gravitational field is spacetime, manifestation of spacetime. So if there's no gravitational field, Einstein would argue, there is no spacetime. He's gone beyond Minkowski spacetime, you know, 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, to switch it off, then you're removing spacetime itself. And if you have spacetime, then you have a gravitational

• 33:11Speaker 03

  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, we're still trying to fit round pegs and square holes by trying to combine gravity in the same way that we're, you know, we dealt with the other, the other forces. Gravity is, the gravitational field is spacetime.

• 33:53Speaker 03

  I came to it because I was trying to understand, is there another way of coming up with the past hypothesis? So, you know, 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, Eddie Chen, who is 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 so some form of quantum entropy, we weren't the first to do this. Lots of

• 34:35Speaker 03

  people come up, you know, have views on what this quantum entropy should be, or 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, you're saying entanglement is increasing, entanglement entropy is increasing,

• 35:19Speaker 03

  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, you know, how or the other attributes of our of our universe, you know, 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, and I find out this is actually really well known by people in the field, that's fine. I don't mind.

• 36:02Brian Keating?

  Well, thank God you, you know, officially retired or emeritus now, you know, because God has

• 36:07Speaker 03

  Absolutely. That's right. I don't have to worry about my CV or public case.

• 36:10Brian Keating?

  Your tenure case would be perhaps a little bit compromised. Okay, I've got three relatively rapid fired 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.

• 36:34Brian Keating?

  So take us through the title, the subtitle, and the cover art, please, Jim.

• 36:38Speaker 03

  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? You know, we'll, we'll come up with something. I have to say, the title, On Time, is brilliant. I absolutely love it. As soon as I thought, oh, yeah, that's, that's it. Of course, the subtitle, The Physics That Makes the Universe Tick, then tells you what it's, what it's all about. But it, you know, it lends itself to humorous comments about, you know, when I'm giving, giving talks on the book, and I make sure

• 37:18Speaker 03

  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 Darley's of melting clock falling into a black hole. No, just have it, 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, I think it's very striking.

• 37:46Brian Keating?

  But I think there's an Easter egg here. The orange is a Clockwork Orange reference. Come on, Jim. It's gotta be a Clockwork Orange reference.

• 37:55Speaker 03

  Hang on, I'm printing.

• 37:58Brian Keating?

  Okay, two more questions. The required, you know, mandated in the state of California, talk about AI. But a lot of times I hear about artificial intelligence in this, 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, you know, I call Einstein's elevator, object in free fall, you know, 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 counter proof that AIs 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, they'll get around that. But, but my question is, can they experience

• 38:43Brian Keating?

  time? Do they experience time? They don't have psychologies. They have some ways of representing memory, but but often it's quite bad. Can artificial consciousness participate in the entanglement that you maybe potentially are beginning to subscribe to?

• 38:58Speaker 00

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• 39:27Speaker 03

  I don't see why not, actually. I think, you know, you're right. And artificial intelligence, just, you know, being intelligent and being able to solve problems that humans can't even come 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 being of experiencing time moving, anticipation of the future, remembering the past, being aware that we live in the

• 40:12Speaker 03

  present moment, I think that requires a sense of knowing oneself, the introspection, sentience. Artificial intelligence is a long way from that. You know, 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, may not feel joy and anger and envy or whatever, and the same we do, but it might become self aware.

• 40:46Brian Keating?

  Very good, Jim. Well, last question, you became a grandfather, I think, since we last spoke. Is that, that's correct, I think. Has time taken on a different meaning since that happened? Is it qualitatively different since becoming a father? You know, they say that, you know, 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?

• 41:10Speaker 03

  I think in lots of ways, and also just, you know, getting older. I remember when I turned 40, that was great. Well, there's no problem with that. You know, I turned 50, I had a big fabulous party. I turned 60, and you think, my God, this is getting serious now, you know. So there's that notion of, 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 are 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'm still don't feel as though I'm, you know, next off the cliff edge, as it were, you know, I still have another generation

• 41:52Speaker 03

  beyond me who's still alive and well. And I think maybe that would change again, you know, when my father passes away, and I feel I'm, you know, next in line. But you're right, having grandchildren does give one a sense of the finiteness of time, you know, 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 feeling that I'm not unique in experiencing.

• 42:25Brian 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.

• 42:33Speaker 03

  I'll do both.

• 42:34Brian 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 here.