Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas - 158 | David Wallace on the Arrow of Time

Episode Date: August 2, 2021

The arrow of time — all the ways in which the past differs from the future — is a fascinating subject because it connects everyday phenomena (memory, aging, cause and effect) to deep questions in physics and philosophy. At its heart is the fact that entropy increases over time, which in turn can be traced to special conditions in the early universe. David Wallace is one of the world's leading philosophers working on the foundations of physics, including space and time as well as quantum mechanics. We talk about how increasing entropy gives rise to the arrow of time, and what it is about the early universe that makes this happen. Then we cannot help but connecting this story to features of the Many-Worlds (Everett) interpretation of quantum mechanics. Support Mindscape on Patreon. David Wallace received a D.Phil. in Physics and a D.Phil. in Philosophy from Oxford University. He is currently W.A. Mellon Professor of Philosophy of Science, with joint appointments in the Philosophy Department and the Department of History and Philosophy of Science, at the University of Pittsburgh. He is the author of The Emergent Multiverse: Quantum Theory According to the Everett Interpretation. Among his honors are the Lakatos Award for outstanding contribution to the philosophy of science. His most recent book is Philosophy of Physics: A Very Short Introduction. Web site PhilPeople profile Google Scholar publications Amazon.com author page

Hey everyone, it's Cal Penn. I'm inviting you to join the best-sounding book club you've ever heard with my podcast, Earsay, the Audible and I-Heart Audio Book Club. Every episode, I nerd out with amazing guests and dive into the best new audiobooks available on Audible. It's the book club for your ears. Listen to Earsay, the Audible and I-Heart audiobook club on the I-Hart Radio app or wherever you get your podcasts. Hello, everyone. Welcome to the Mindscape podcast. I'm your host, Sean Carroll. The passage of time, it gets to us all, doesn't it? I've been doing this podcast for over three years now. I remember almost everyone that I've done. Actually, I remember everyone. But I don't remember any of the ones that I haven't yet done. There's an asymmetry, right?

In my memory between the podcast that I've already done in the past and the podcast yet to come in the future, not because there aren't any in the future, but because of some feature of the way the time works in our universe, the arrow of time pointing from. the past to the future, that gives it that imbalance. And this is one of my favorite topics to think about in physics and philosophy. And the amazing thing is, we haven't really talked about it here on the podcast yet. We did talk about time, the psychology and neuroscience of time with Dean Bonamano,

but the physics and philosophy of the arrow of time, which was the subject of my first trade book from eternity to here, we haven't talked about yet. Also surprising is that we've not yet had David Wallace on the podcast. David is one of the leading philosophers of physics in the world today. The third surprising thing is that we're having David on and we're not talking that much about the many worlds interpretation of quantum mechanics. David is most famous for being probably the person who is thought most carefully about how many worlds should work in the real world. I all encourage you to check out his book, The Emergent Multiverse, if you really want to dig into the details. But David is broad in his interests in physics and philosophy.

and he's certainly thought a lot about entropy and the arrow of time and the initial conditions of the universe and so forth. That's what we're going to be mostly talking about today. How are all these different things related in the typical good philosopher very, very careful way of teasing out what all the assumptions are, how they go into things. I know that I've found by talking to people that the arrow of time is one of the hardest things to think about in a reasonable way just because this idea, that the past flows into the future is so deeply embedded in how we think about the world that the idea that it's something that has to be explained and that you're not allowed to cheat when you explain it is a hard one to wrap your mind around. But David is very, very good at it. And guess what? By the end of the podcast, we are in fact talking about the Everett Many World's interpretation of quantum mechanics because it has a relationship to the arrow of time. The universe branches into multiple copies of itself going forward in time, not going backward.

So I think this is a classic upcoming episode here of the Mindscape podcast, and you're all going to enjoy it. So let's go. David Wallace, welcome to the Mindscape podcast. Great to be here. You know, we've had some philosophers on the podcast before. In fact, philosophers of science. In fact, philosophers who started out as physicists and then turned into science, like our friend David Albert, for example. But nevertheless, I thought it would be fun to start by just asking you about that transition, you know, very quickly before you get into the mean potatoes of the thing.

What is it that causes a person to sort of at one time in their lives think, I want to be a physicist and then at some other time think, you know, I'd really be happier doing philosophy? Well, I think there's a bunch of people who get into physics and then gradually start thinking that a more philosophical way of exploring things. is more interesting and they want to move in that direction. I'm not that kind of person at all. I still want to do physics. Largely, I still think I do physics. The kind of work I do is interdisciplinary and I think relatively mundane facts about the way

Britain and America organize their universities mean that for the kind of physics or interdisciplinary stuff I was interested in, staying in a physics department and trying to do that work wouldn't have been the smartest career move. Right. So I kind of finished my PhD in physics with a bunch of research interests that were kind of in various foundational things, quantum field theory, the Everett interpretation.

And it looked pretty clear that I'd probably be able to get postdocs that way relatively easily because everyone says they love it to this blue work. And when I started looking for tenure track positions, I'd be short of luck. Right. So it was a fairly practical career move. I don't understand how cool academic flow. philosophy is or say that I haven't developed lots of interests beyond that through exposure to it, because I really have. But it really did come down to that kind. I want to do this kind of physics.

Where can I do it? How can I do it? I mean, it is a indictment, I think, of the academic system, at least in the UK and the USA, which is the ones we're familiar with, that the siloing is strong enough that there isn't a spectrum in between these, right? I mean, you are either a philosopher or a physicist because you need to get paid. You need to get a job. And that affects how people choose their research topics and things like that. Yeah. Yeah. And people like it to disciplinary work in principle, but when it starts to be, you want a permanent position, you have a teaching component, it gets hard. And I do think it's a genuinely hard problem to solve. I mean, one of the reasons foundational, philosophical works tricky to do in a physics department is the

criteria to measure it a hard. So you've done an experiment. The experiment does what you said it did, you've done a calculation, the calculation can be checked. All that's fine. Harder to do that with foundational stuff? I mean, as we both know, some genuinely bad stuff gets done in foundational spaces, which doesn't get caught by those kind of filters. So I think it is an indictment, but I also do think it's a tricky problem to solve. And maybe one reason philosophy of physics works quite well as a space is philosophy is a bit better and knowing how to assess for quality that kind of more conceptual-oriented work. That makes perfect sense. And actually, it leads very well

into the meat and potatoes here because my own, I think I've said this before, but maybe not to you. You know, I was always interested in philosophy ever since I was an undergraduate, but the kinds of philosophy that I liked were actually moral and political philosophy because the philosophy of science that I learned was all about theory, choice and sociology of science and stuff like that, which is important and interesting, but it's not my bag. It wasn't until I was a grown cosmologist and thinking about the early universe that I came across a bunch of philosophers who were really, like you say, really doing physics, right?

People like Hugh Price and David Albert and Tim Walden and yourself who were trying to understand how the universe works. And it was through the arrow of time that I actually got interested in philosophical, foundational questions. And so I thought it would be fun to start talking about that before we get into the Everett interpretation. So the phrase, the arrow of time, something people have heard, but then there's this feeling that there are many arrows of time.

I mean, what is your take on? Is there an arrow of time? Are there many? What are they? How do you define them? The basic way I'd started getting at it is something about which processes in the world run forwards and backwards just as happily and which processes don't. So if you look at small systems, or not really even just at small systems, but if you look at systems with not many moving parts, in physics, they don't seem to care whether you run them forward. or backwards. So if I showed you a video of the solar system and I speeded it up a bit and I asked you whether I was running it forward or backward, I mean, if you paid attention to whether the sun rose in the east or the west or something, you could probably figure it out. But it wouldn't be obvious. The solar system's physics doesn't really mind if you run it forward or backward. And that's just the same reason why the way we work out when the next eclipse is going to be is

just the same way we work out when the eclipses were in the classical era so we can cross-reference it to Thucididides or something. Most processes aren't like. that processes with lots of moving parts I don't like that if I show you a speeded up video of ice cube in water or all the way I've got I've got older over the last 10 years or you know a building collapsing or something it's completely obvious whether I'm running that video forward or backward so most processes in the world really distinguish between forward and backward dynamics and yet big things are made out of small things so there's something extremely puzzling, kind of in the vicinity of a contradiction if you're not careful,

with the idea that big processes clearly distinguish forward and back with the dynamics, but they're made out of processes that themselves don't distinguish it. Do you yourself like to, you know, rigorously categorize different arrows of time, or do you just lump it together as an arrow of time and different aspects of it? Yeah, I'm kind of skeptical about a lot of that categorizing of different arrows. I think, you know, people do talk about a statistical thermodynamic arrow and a cosmological arrow and memory arrow. And I think the way you break those things up kind of presupposes structure that's not necessarily there. I don't think we should necessarily take it as read the direction of time implied by the fact that we remember the past, not the future, has the same origin as would be assumed as the direction we get for,

the fact that ice melts and doesn't re-freeze. But if we just kind of silo them off, then we can kind of assume that we know how those things break up in a way that isn't completely reliable. And I think one way of seeing that actually is you start, if you want a check of two things are logically independent, you want to try switching one of them off and leaving the other one on and seeing if it makes sense.

And if you try to imagine universes where you turn around, let's say, the direction in which you remember things, or the direction in which radiation decays, but you don't turn around the direction in which ice melts. You very quickly have something you can't really imagine when you start trying to think through the details in any length. So, in other words, just to sort of leap ahead to a big claim that we'll interrogate later, you seem to be saying that in your view,

there is one underlying thing that ultimately is going to account for both why ice melts but doesn't unmelt and also for why we remember the past but not the future. Well, look, I reckon that's true, but I'll do a slightly more, sort of drawn back version for that. I'm pretty sure that the reason that if there's a relation between those two, then the direction of time in human cognition and action is what it is because of the direction of time in physics and not vice versa. So we ought to be able to figure out what's going on in the direction of time in physics without having to presuppose a bunch of stuff from human cognition and action. If once we've done that, then I kind of strongly suspect that,

that because humans are physical systems, it's going to turn out that asymmetries and humans are a special case of these other asymmetries. But at the very least, we can figure out those asymmetries in physics without having to get tangled up in these issues about asymmetries in us. So this does presuppose, and you're in a friendly crowd here, so it's okay, a kind of physicalism, right?

I mean, we're not attributing human agency to something over and above the fact that we're made of stuff. So if we understand the asymmetries, the behavior of the stuff, the asymmetries of human thought and cognition should follow along. Right. Yeah, absolutely. There's a substantial assumption going on here. And there's a sense in which I'm not even seriously defending it. It's a bit too foundational start defending. You might kind of draw it up like this. There's this kind of approach, I think you're calling physicalism, some places I've just called it like dynamics first that says something like, okay, the way we manage our science is it's a

dynamical description of what's going on, independent of us, insofar as we interact with it, that matters because it's how we test stuff and because we want to understand us too, but the physics itself can be understood without and without consideration of the agent's role. And there's a completely different tradition that says, well, the theories can't be understood except insofar as they are things to be used by theorizers. And the role of the theorizer can never be taken out completely. So you'll never really understand the asymmetry of time. in physics without understanding that there's an asymmetry in how we as agents get at systems.

Look, some smart, reasonable people take that approach. And sometimes it bears fruit. And there comes a time where I think it's less useful to just go around and round and never decreasing circles, try to persuade somebody who has that starting point that they're wrong. And it makes more sense to pursue your own project and have it judged by its fruits. That's right. If you succeed, then good things will flow in your...

direction, right. Okay, so I mean, you mentioned, I think, very correctly that naively this asymmetry of time evolution of big complicated things might be thought of as intention or even contradictory to the symmetry of time evolution in things with small number of moving pieces. Before we explain why, do you know much about the history? Like, who's the first person who pointed out this was a puzzle? So I should know this. Lots of philosophers of physics are really good at the history of physics. I'm not one of those philosophy physics, but I hang out with them a lot, so I can do my attempted secondhand version. I mean, back in the late 19th century, when Boltzman was first tried to write down mechanical theories of gases, these ideas were in the air and developing, and there was clearly a lot of confusion. And it's one of these quite delicate things to tease out exactly who knew what when. But roughly speaking, Boltzman had a stab, but doing an entirely mechanical explanation of why gases expand, expand. and expand to fill the space they're in and why they approach an equilibrium, a kind of stable, well-behaved distribution of a certain kind. And he kind of, at least on the natural reading of it,

he wrote a paper that kind of tried to just prove this mechanically. And then a bunch of people's, a mailer, Lo Schmidt, pointed out like, this is logically impossible. You started with some time reversible equations. You made some time reversible assumptions about those equations and you got out something which distinguishes the past and the future. This can't be done. And also you started with, this is a slightly subtler point, but you started with some equations which reliably say that wherever a system starts off, it will eventually get back there, and you've derived the fact that the system will start somewhere, go somewhere else and stay there forever. This cannot happen. So I think really very early on, you know, before statistical mechanics was a properly developed

discipline, right at the very first attempts to do this stuff, then people were very quickly seeing that there was an issue there. I don't think it was much before Boltzman because you had to have a realistic and detailed understanding of how the large came out to the small to get the worry off the ground. I mean, I think Newton kind of liked the idea that there might be like an atomistic basis for everything, but it was so embryonic at that point that I don't think anyone's serious, that I'm aware of, seriously tried to think through what the possible issues were there. Well, I guess one of the reasons I'm asking is just because I don't know the answer and I've kind of wondered about this.

I mean, I don't want to, you know, say too many things rather than asking you to say them. But there is the second law of thermodynamics, which roughly says that entropy increases, and we can get into that. But it predates Boltzman. Like, Boltzman gave an explanation for it in terms of atoms and molecules bumping into each other. So I've always wondered, you know, once they did come up, you know, Carnot and Clausius and people like that, they invented the second law, manifestly not time reversible, right? mean, it could be increases with time. It doesn't decrease. And they also knew that there were Newton's laws of motion.

Like, that was well known. And so did anyone, was the idea that things like the second law were over and above Newton's laws of motion? Or was there even a hope of deriving them from the start? And maybe the answer is you don't know, because I certainly don't know. And I've definitely tried to find out. I mean, the sense I have is that the apparent problem there just doesn't bite unless you've a kind of broadly in the good sense reductionist picture of what's going on in the world. I mean, if you think that Newton's laws are supposed to be laws for everything,

then you can see the contradiction is going to turn out pretty fast. If you think Newton's laws is a great for planets and cannonballs, and they're not great for living things or for clouds, then you're not going to be so bothered. You'll have a fundamentally pluralist world you're only beginning to understand, and you don't know how all the bits connect together, and the fact that the bit that describes small mechanical stuff has a symmetry in time,

and the bit that describes steam engines doesn't have a symmetry time, isn't going to bother you. And we can look back now and think how inevitable it ought to be that these things get put together. But you've got to remember, even in the early 20th century, people were still taking extremely seriously the idea that living tissue was not described by the same laws of physics that described none of living matter,

that there were ground-level laws of nature, of nature that applied to living tissue that didn't apply to an un-living tissue. And it wasn't even a stupid thing to think. I mean, it was always a little unimaginative, but they didn't know about quantum mechanics. So I think the discovery that the world actually is really pretty unified and that in the good sense, it's pretty reductionist, is a really substantive, important discovery of 20th century science. We don't want to just read it back as some automatic a priori fact about science that they could have known just by cognition in the 19th century. So that's my sense of it. It's only when Boltzmann actually writes down something that starts looking like a

proper mechanical description of steam or gases, and a bit more early than that when Maxwell's starting to do similar things. It's only once people are starting to actually put in the connective tissue between mechanics and thermodynamics and starting to get results that make it plausible that they're connected in this way, that you start seeing the bite of the tension between. That's my sense of it, but again, it's historically not super informed. No, but actually it makes a lot of sense in it. I like it because, if anything, it elevates the accomplishment of Maxwell and Boltzmann and their friends. I mean, it's not just that they helped establish statistical mechanics, but reductionism or even the unity of science in some sense, right?

The idea that human beings are made of the same stuff and obey the same laws that Adams do is exactly as you say, whoever came up with it, it's a non-trivial step. Yeah. All of us are trying to get in shape and eat right. Freshly is here to help. Freshly's delicious meals are designed by nutritionists and cooked by chefs, making it easier for you to eat better. Freshly is not a meal kit that gives you ingredients that you have to cook. Freshly offers chef-made, nutrition-packed delicious meals delivered fresh to your door. Your meals arrive cooked and fresh every week so you can keep your fridge stocked and skip the trip to the store. One of the things that I like most about it is that there's no waste.

You don't buy food and then not use it, because everything that you get in the package is something that you will eat during that meal. Your meals are always delivered fresh, never frozen, and are ready to heat and enjoy in just three minutes. With new meals added each week, Freshly brings the convenience of chef-made, nutritionist-designed classics right to your kitchen. And right now, Freshly is offering our listeners $40 off your first two orders when you go to Freshly.com slash. Mindscape. So stop stressing about dinner. Go to F-R-E-S-H-L-Y.com slash Mindscape for $40 off your first two orders. That's freshly.com slash mindscape. And so then this leads us into what Boltzman did. So how, you know, what was the insight into entropy and disorderliness and time that Boltzman and his friends came up with? Well, again, abstracting a little bit from how, how that kind of very messy

history to her now. The history is not our primary goal here. What they started, what they started to realize was something like, okay, it's not, it can't be that you could just logically derive the asymmetries, the expansion of gases, the second law in your example, from, from these mechanics. You have to put some additional assumption in. And the additional assumptions they put in, or at least one of them, one of the ones that gets a lot of the traction, was some notion of probability. It was some idea that, okay, the gas can't, you can't be sure that the gas will expand to fill the box, but it's extremely likely it's going to. And to some extent, their notion of extremely likely gets cashed out combinatorically, like how many ways could the gas

be? And like almost all the ways the gas could be in this cashing out are ways which is going to expand. That's kind of a bit dodgy because there's like infinitely many ways the gas could be. and so comparing the numbers is questionable. But that kind of basic idea was there, that what was going to solve the problem was going to be probability. And the funny thing is that that lesson actually, I think that idea that you get round this paradox

by introducing probability is still, I think, quite often what you run into in the kind of second year undergraduate statistical mechanics course that wants to spend the first lecture during the history of the subject so it can spend lectures 2 through 16 teach you how to calculate partition functions. But you step back for just a moment and of course it can't be true by itself because

leaving aside entirely some really deep questions about how we understand probability in this context, the mere introduction of probability doesn't break the time reversibility. If it's if you want to say it's overwhelmingly probable that the gas is going to evolve to fill the box in the future, but you've done. don't want to say it's overwhelmingly probable that the gas is going to have been such that in the past it filled the box, then somewhere you must have broken the time in symmetry. And so the time symmetry. And appealing to probability by itself doesn't change that paradox at all.

And I think actually, I just want to dwell on that statement because it was beautifully put and it's at the heart of a lot of difficulty, I think, that people have with the arrow of time and with related notions because they come up with some. statements, some purported explanations that all sound good. But even though what they're trying to explain is the asymmetry of time, they don't go through the exercise of running that thing backward in time and seeing if it sounds just as good. Absolutely. Now, I have an article I this a while ago said, computer scientists say garbage in, garbage out. If your input is a mess, you're going to get out, your output's a mess, however clever your program is. So the slogan in

statistical mechanics, or philosophy of statistical mechanics, or be reversibility in, reversibility out. If the assumptions you make don't break the symmetry of the problem, they don't distinguish past the future in some sense, it doesn't matter how clever your argument is. If you get out, you will either fail to get out an asymmetry at the end of the argument, or you will have cheated somewhere and smuggled in the asymmetry, possibly without even noticing it yourself,

but you'll have smuggled it in, just as a matter of logic. You can't have an input that doesn't distinguish a direction of time and have an output that does distinguish it. So what do we do? What is the right answer? Why is there a, where does the asymmetry of time come from then? Okay. So, well, I think you can think of three strategies. And one of them we could probably put aside, which is this idea that it comes from, it comes from something nonphysical. It comes from human intervention or something. So I think for our purposes, we can, we can put that aside and continue with us in our physicalist dogma. within physics, if you're going to break that symmetry, I think you can only really logically, again, you can only be in two ways. Your dynamics needs to violate the direction of these to distinguish past and future or your boundary conditions need to distinguish paths from future. And so the former approach needs you to change deep physics.

Put that aside for the moment. That's another conversation we can have and it gets on to somebody going to. I think we'll talk about later, which is how quantum mechanics plugs into these questions. Assuming you don't want to change the microscopic physics, you've got to say something about an appropriate initial condition. So, and what often happens is what you end up having to do is say something that gets pushed early and early in time. So let's suppose I do one of these really innocuous sounding statements about it's really probable that the system will approach the gasol spread outlet. it was really probable that entropy will increase, etc. If I place that condition now,

if I say it's a condition on the world now, that with really high probability, entropy is going around to the future, or more plausibly, if I have some condition that looks really innocuous and has that consequence. Well, again, as a matter of time symmetry, it's going to have the same consequence backwards.

And the forward result is what we want and the backward result is what we don't want. So you could try to do it earlier. You could say, maybe I'll impose a boundary condition 100 years ago that says, which has the consequence that we would expect gases to expand and entropy to go up and so on. Now the last hundred years is fine. And now everything we think is, we thought we remembered about everything prior to 1921 is completely wrong.

So we could put it back a million years ago and that saves human history, but it's too bad about the dinosaurs. We can put it back a billion years ago, which is the dinosaurs are fine, but it's too bad about formation of the solar system. eventually you end up saying I need to, again, if we're sticking with this sort of broad picture, we want to be reductionist, we don't want to rely on external intervention. The only really place,

the only real option you've got left is to put some kind of relevant boundary condition at the beginning of time. So then you say, well, the universe, or at least the bit of the universe, we were aware of, the kind of observed cosmos, came into being, or very early on was such that,

it has these features, whatever they are, and that's another conversation, which establish that entropy goes up into the future. And then you've kind of just, you just kind of crossed out the earlier bit so the paradox doesn't occur. There's a kind of sense in which you, you haven't exactly broken the time reversal symmetry in physics jargon, the distinction from past and future. You've broken the time translation signature, symmetry, the idea that there's a, there's no preferred moment. In a sense, you're using the first moment as your preferred moment. I've always... Go on, sorry. I've always wondered, again, an unfair historical question. Why didn't Boltzman or someone in his vicinity propose the Big Bang? Why didn't they take seriously?

I mean, he did in a paper suggest that one way out of these reversibility problems was to have a special initial condition. But it was really like a sentence in a paragraph and then he moved on with his life, right? And no one quite chased that down and said, look, you know, stars are burning some kind of fuel. We don't know what it is, but presumably you can't go on forever. There must have been a moment in the past when things were set up in a special condition. Yeah. So historically, I have no idea. I mean, trying to do a rational reconstruction, I think part of it is, until you've got the general theory of relativity, any decision that a particular moment is the first moment is just a labeling choice. If you, you, it's a, it's a, you, it's a,

You can imagine saying, well, this is time equals zero. This is the first moment. It is forbidden to run the dynamics backwards from the first moment. But then your graduate student says, well, look, it's free country. I'm going to run the dynamics back anyway. And then you've got a perfectly reasonable description backwards. And you've got effectively a symmetric cosmos that has a big bang in both directions, which is an offensive, you know, I gather some quite smart people have played with that idea.

But it's not the Big Bang idea exactly. I think it's when you've got something like Yemen relativity on the table, you've got the idea that the theory might enforce the fact that a moment is first. Or as you know, not literally that a moment is first, but that there's a point beyond which you can't mind. There was some special moment, yeah, in the history of the universe. I think that's part of it. But I think also in a way I don't entirely understand,

there was clearly a really strong philosophical prejudice against the Big Bang. I mean, the advocates of steady state cosmologies did so. Quite a long time after the empirical case for it was looking shaky. They were clearly very philosophically motivated for it beyond the data. As everyone who knows the history of this at all knows, Einstein could have predicted the Big Bang, and he instead cooked up his equations to stop it happening. There was clearly something in the air I don't understand

that made people really skeptical about that kind of beginning of time concept. And this is this idea that we need to set up the initial conditions in a certain special way. I mean, number one, it truly blows my mind even to this day that the reason why ice cubes melt and don't unmelt has something to do with what was going on 14 billion years ago at the Big Bang. So, like, I get a lot of mileage out of that, and I'm still impressed with myself. And number two, there's this label for this idea, which is the past hypothesis. And then we argue about what the past hypothesis really means. So what is, in your mind, why do we call it the past hypothesis and what is it? Okay. So the easy answer to why do we call it is because David Albert is good with terminology

and came up with this name or popularised. I'm not sure. It's mostly his language, I think. So that's the shallow answer. It's some, well, in the middle version, we're just saying something about the initial universe. Some of the reason it gets talked about as a hypothesis is there's a kind of tradition of thinking that somehow this assumption about the past, is not supposed to be the kind of thing we might make in the same way we know other things about the past. I mean, somehow it's not just that we put up a satellite, and the satellite told us something about the early universe, and so we're using that as our boundary condition.

It's supposed to be something more that it's a presupposition for the applicability of the physics we've been using, including the physics the satellite assumes that the very early universe has this feature. in terms of what that feature is, I mean, that's contentious. I mean, you know, you and I have been on different sides of some academic conversations about this. I mean, a very popular idea is the condition is something like the very early universe has a very low entropy, is in a very special state in the probability sense I was using earlier. There's another sense of which I prefer, which is actually something like the, it's more that the very early universe is in an appropriate sense, not in a special sense. not in a special state. It's in a state that doesn't have the sort of very delicate, precise correlations between where all the particles are that would mean that you weren't

in one of these probable conditions whereby entropy increases. So just how to cash out this condition is a pretty subtle matter. I mean, you can kind of work out just by logic. There has to be some condition of that kind if you're not, if you're not breaking the symmetry in the dynamics, you've got to break it in the boundary conditions. Right. But quite, what the right way to break it is. Yeah, the devil's going to be a little bit in the details. I mean, maybe this is actually, it's always fun, but also dangerous

to get too much into the weeds. But maybe there's a weed or two that we can get into here because it's worth driving home because even professional physicists struggle with this idea. One way of stating the issue is that, look, we have satellites up there. They've taken pictures of the early universe. right? The cosmic microwave background. It's very smooth, you know, almost homogeneous,

but with tiny little ripples in it. And we explained that in terms of a smooth Big Bang 14 billion years ago. And let's take for granted right now the idea that that condition of a hot, dense, smooth, or the universe is low entropy. We can talk about that too. Let's say that is low entropy. So there's an attitude that would say, look, we've taken a picture of it. It's not a hypothesis. we have data that says it was low entropy, but then the much more subtle counter argument is, in the space of all possible ways that microwave photons could have come to our telescope

and looked smooth, it turns out that most of them do not correspond to an early universe that actually is smooth, right? There are many, many more ways the early universe could have been wildly fluctuating, but with very, very subtle coincidences and conspiracies that made the radiation get too, us and look smooth. And it's that kind of perverse behavior that you in the past

hypothesis are trying to rule out. Is that fair? Right. I think that's fair. Although, let me give a caveat because, I mean, you are helping yourself a moment to go to this kind of, well, there's vastly more ways it could be this than could be that. And I was using that kind of way I talking earlier. But actually, I think if you push on this, it starts getting into trouble. I mean, I said sort of flippantly, well, there are infinitely many ways the system could be, so there's no sense of which could be more one way than another. But that is kind of true in a way. And even if there were like a million billion,

caesillion, cajillion ways the system could be, but it's finite, it doesn't follow from the fact that there are a million, zillion billion, cajillion ways a system could be, that it is equally likely to be each of those ways. I mean, I think we've got a ton of evidence

from the actual application of physics that there's some robust notion of probability in statistical mechanics. But I also think we've got quite a lot of reason to think that that notion of probability is not something we can get at a priori from just counting possibilities. Okay. There's something substantive additional going on.

The assumption is roughly something like, for a given macroscopic way the world could be, then it's, again, leaving aside my caveat about infinity, each of the microscopic ways it could be conditional with that macroscopic way it could be is equally likely. That's something like the way of stating it. Yeah. But it's a, it's quite, a big step. That's something I think is empirical evidence for, at least with some qualifiers and caveats and worries about the direction of time. But at some level, there's empirical evidence for it.

When we make that assumption and we derive equations as a result of that assumption and we check the experiments in the lab or in the sky, the equations get the answer right. So that probability assumption is evidenced. We don't need to think that is our priority. The view that somehow, if there's lots of different macroscopic ways a system could be, then it's more likely to be in the macroscopic state that has more microscopic states corresponding to it. Well, there are special circumstances of which that's evidenced as well, when systems have reached equilibrium, when they've had lots of time to interact and move around the space, then there's evidence that's true. But as a kind of a priori statement about the world, I don't think it's something we've got

evidence for or a good logical reason that it has to be true. So while in a certain sense, you're clearly right that there are many more ways, it's much more probable that the universe fluctuated into the way it is now rather than the satellites gave us the right answer. There's another sense in which that's none innocent, that there's a, there are some assumptions about how to think about probability that are, that are doing some work in that argument. Long time Mindscape listeners know that our two cats, Ariel and Caliban,

are the real bosses here at Mindscape International headquarters. So we need to keep them happy and Sweet Scoop is the kitty litter that makes that possible. The thing about SweetScape is not only that it works as a kitty litter, the cats love it, the odor is good, the absorption is good, but it's also environmentally friendly because sweet scoop comes from wheat rather than from clay. Wheat, it turns out, has naturally occurring starches and enzymes that actively break things down. It's low dust, low tracking, and easy to dispose of, and unlike clay-based litter, sweet scoop is 100% biodegradable. For over 25 years, Sweet scoop has provided a litter that works great

without compromising on what matters, 100% natural ingredients, and 99.5% dust-free. So if you're stuck between litter that works and litter that's pet and planet friendly, sweet scoop doesn't make you choose. Go to sweet scoop.com to get a $5 off coupon and find a retailer near you.

To get the spelling right, that's s-w-h-e-a-t-t-t-scoop.com. Sweet-scoop.com. I think that's perfectly fair if maybe a little bit persnickety, but that is your professional job, right? I mean, you're a philosopher, so that's okay. It's pretty much a job description.

Yeah, exactly. We're not going to hold that against you. But so let me just rephrase it very quickly just so, you know, we keep separate, like, what the contentious parts are and what the parts everyone agrees with are. I mean, you're probably right that since my job description is physicist and not philosopher, I speak sloppily. That's my job description, right? So I use the word probability where I really meant, you know, number of different states or something like that.

I do think that there's that philosophers can sometimes be too persnickety about counting the number of different states. I think there's a perfectly obvious way to count the number of different states. And that doesn't mean that it's a probability distribution. That doesn't mean that, you know, in Penrose's famous formulation that God threw a dartboard randomly at that space of states. But there remains the fact that the past hypothesis needs to have some way of saying that in the space of all the different ways the universe could have been, there's a measure, there's a wave counting, and our real universe was in a very, very tiny region in that space, and that needs to, that's the past hypothesis broadly construed. Yeah, okay, so I don't think this is just, just penitry.

I think we actually disagree with something substantive here. I mean, so there's a well-known way of state in the past hypothesis that says something like you said there, that the early universe, the hypothesis is that the early universe was in a very low-ed-d entropy state or a certain particular state that is in fact very low-endribus. and that if one assumes that, then one gets the direction of time, right? I actually think that's the wrong way to state the hypothesis. And one way of getting at that is I think this is like a minor bit of it. I mean, however low entropy the initial universe was in,

it could have ended up in an even lower entropy state later. The issues about time reversibility are still there. I mean, let's suppose that the early, well, okay, So firstly, actually, let's say the early view this was very hot and very dense, and we've established that we think that's a low entropy state. It's still a sense in which, you know, it could have been hotter and denser than it would be low. Or it would be hotter and it would be lower entropy. So there's a certain worry there.

But more seriously, suppose that was a very low entropy state. There's still a way in which it could have fluctuated to an even lower entropy state. So we don't rule out that possibility just by specifying this very low entropy initial state. We have to say something else. And there's something else, I think, takes the form, actually helped myself to the physicist sloppiness, that given that initial state, it was equally likely to be in any of the microscopic states corresponding to it. And so it was super unlikely to be in one of the fluctuating to an even lower entropy state. Yeah. But my feeling is once you've said that, once you've said, okay, given the macro state of the early universe, it was equally likely to be in any of the microstates compatible with it.

I don't think anything more is gained, at least for the purposes of explaining the hour of time in the here and now, by saying, and what's more, that state was a really low entropy state. I mean, if it was a really high entropy state, the world would not look like this. But it's also true that if it was a state with a different kind of batter, anti-matter ratio, it would also not look like this. The smoothness of the early universe explains lots of things about the here and now, but I'm not compelled by the argument that it explains the time directiveness of our,

macroscopic physics. So let me see. So in other words, you're saying, so actually, let's back up a little bit. There's the idea of a macro state, which we sort of glossed over, and people want to make a big deal about this, and I think it's proper to make a big deal about this. So one of Boltzmann's genius moves was to say, look, you know, there's atoms molecules, they're moving around, but I can't see what the atoms of molecules are. So I'm going to group that all the atoms and molecules that look similar into a single group that I'll call a macro state. I don't know if he used those words, but that's what we do? And so people have been asking, so I will ask you, like, what's the license to do that? Is that okay? Is that objective? Or is, you know, human

agency sneaking its way in here? Something's subjective? Good, yeah. I mean, the words get used in two kind of different ways, and I think one of them should worry reductionist, naturally. So one of them basically shouldn't. And the one that should worry people is the one that defines macroscopic in terms of our own abilities to distinguish them. So you want to say, well, the reason why we treat two different configurations of gas in the room as being the same macrostate is something like we can't distinguish between them with our eyes or sometimes even like we're only interested in the gas at the level of a coarse graining, you know, like a like a macroscating. You know, like a like a acroscopic description. But, you know, I mean, hashtag embarrassing confessions, I don't actually

care about the gas at all. With beyond the minimal requirements to be able to breathe, I don't really mind where the air in my room is. It's still true that thermodynamics describes it fairly well, even though I'm not curious about it. And similarly, we use statistical mechanics to describe even things like the way galaxies and stars in the galaxy develop. And we can obviously just, you know, our telescopes can tell us where the damn stars are. And yet we still want to. to dump various clearly visibly distinct arrangements of stars into the same macro state and say what we care about is the average distribution of the stars or something. So I think the good notion

of macrostate, the one that the naturalist can be relaxed about, is something like this is a level of description at which you can get some kind of robust, autonomous in the philosopher's jargon sense, self-contained, dynamical story. So it's a, if I, If I want to know how the air in the room is going to revolve over the next five minutes, at a certain course grain description, it turns out that all I need to know about the air now is, again, at a certain core, well, modulo these worries about thermodynamics. So it's just empirically it's true that I can make reliable predictions

about what the air will do at a certain course level, provided I know what the air is doing now at a certain course level. And, you know, we have equations to say what water does that don't require us to say where all the individual molecules are. We can do higher-level science. And the macrostates are the way of breaking up the lower-level stuff into higher-level stuff such that you can do high-level science. That's the robust version, I think, that we shouldn't be suspicious about. Yeah. And I think...

Go ahead. Sorry. I was just going to say, I mean, that feeds into not just statistical mechanics, but bigger issues of emergence and Dan Dennett, former Mindscape guest, has this wonderful phrase, Real Patterns that you also, that you make use of in the case of the Everett interpretation. I mean, the fact that you don't need to know absolutely everything about the universe, but you can have incomplete information and still in the right circumstances, make predictions about it, is an objective fact about the universe,

and that's what we appeal to in this sort of Boltzmanian core screening. Yeah, absolutely. And it's a solid effect of the Meductionism itself. I mean, it's just implicit. It's theoretically true that we can write down reliable equations for what gases do. Didn't have to be that way. Now, you know, if you can, often we found those equations because we did microphysics and worked it up, but not always.

And even if we've done the microphysics and worked it up, we could have thrown it away and still check the equations and they were still right. Right. So, you know, insofar as your best theory of microfysics says it's not possible to have equations of that form, so much the worst for your best theory of microphysics. Our actual evidence for our microscopic physics mostly doesn't come from microscopic measurements. Mostly it comes from these millions of intermediate calculations. We talk about the standard model explaining everything or underpinning everything.

There's almost no experiment that counts as a systematic test of the standard model as a whole. As you well know, there are approximations and chains of approximations that ground all the various ways these theories work. And the first order evidence is the evidence that these various approximations are right. And because they're right, then that gives some support for the theory for which we derive them. But really, the world we find empirically in the first order is a world just full of all of these different processes, different levels that we think are connected. And we've got, I think we've got a good reason to think are connected, but which we have evidence for even if they're not connected. Good. And specifically, so there is some sort of objectiveness to the course screening

we choose in practice, and specifically because some of my favorite super smart cosmologists don't understand this, the early universe has a low entropy in that coarse graining. There's this feeling that you say something like, look, it's just a black body radiation. That's high entropy. You know, how could it, it had the highest entropy it could have had given that it was tiny. But when you turn on gravity, that's not true. So am I right to say that it's fair to say the early universe had low entropy compared to what it could have had? And gravity has something to do with it.

But here's something I don't understand about this in these conversations as cosmologists. I mean, absolutely, the early universe had low entropy compared to what it could have. But one really quick way to tell that is, I mean, the early universe had low entropy compared to the current universe. I mean, if it didn't, the second law of thermodynamics is empirically wrong. And no one thinks that. Well, yes. So it had really better be the case that the early universe was lower entropy than the present universe. We shouldn't need to give some subtle transidential argument as to why it must be lower.

It had better be lower in order not to blow our entire thermodynamic theory out of the water. But to be fair, the argument, which is completely wrong, but still the, it's not completely nonsensical, is that there's sort of an envelope of maximum possible entropy just because the universe was smaller. And sort of it was the argument is that it was as high entropy as it could. have been, even those lower entropy than now. But even that argument is totally wrong because it ignores gravity in black holes and things like that. Yeah, although I mean, it does, but I'd also say, look, it actually ignores the fact the universe is expanding. And I think even that homogenous piece of it in some ways, like smooth, is enough to do the work we need of

it here. I mean, the very early universe was in this equilibrium thermal state. Then it expanded. And that was the highest entropy it could be, given the other constraints on it and given that it was uniform. Given that it was uniform is a huge constraint. Absolutely. But look, go forward a little while and it's still uniform, but it's no longer the highest entropy it could be. There comes a point where all the quark gluon slush has turned into neutrons and protons. And there comes a further point when some of the neutrons and protons are fused into helium. So there comes a point when it's about three quarters hydrogen and one quarter helium.

And then it stops there. And it carries on being three quarters hydrogen and one quarter helium. If it had wanted to stay at equilibrium, it wouldn't have done that. Right. The equilibrium condition for the universe, this is more your territory than mine, but like well, well before it gets non-homogeneous on large scales, it was, it was, the equilibrium configuration would have been all helium. That's right.

and it wasn't all helium. Why? Well, it was expanding so fast that the particles got too far apart before they finished crashing into which enough to make helium. So, I mean, I just used that as an illustration. Well, you're absolutely right, of course, about the fact that gravity and things not being uniform mean that the real story about entropy in gravitating systems

is just radically different from ungravitating systems. But even at a more basic level than that, trying to apply kind of equilibrium arguments to a system that is very rapidly expanding is itself really dubious. Really rapidly expanding systems are not at equilibrium. The universe's macro conditions were changing very fast. So it was only ever reasonable to suppose it was some kind of local equilibrium. There's a, you can think about the very old universe while it's still uniform as being

sort of like a piston. If I put some very hot gas in a piston and I expanded it very gently, then the gas would always stay instantaneously equilibrium. and if your piston was like ridiculously strong, you could compress it, you could expand it from being a quark glue on plasma all the way to being cold iron and it would all be the equilibrium.

If you pull the piston apart a bit faster than that, then it'll freeze out at some mixture of stuff that hasn't finished going to equilibrium. And it did that because it was expanding and it was expanding too quickly. And so I think those things are going on of the only universe, even before you start considering

gravity and effects of, you know, gravity and none, you know, none uniform distributions. And when do you think that the, the earth is powered by fusion from the sun and the fusion for the sun is possible because there was lots of hydrogen left over from the Big Bang, which wouldn't have been left over

if the universe had stayed at equilibrium as long as it was uniform, then you realize that the thermodynamics of the present day actually rests on some of these factors as much as it rests on issues that non-uniforms, systems of higher entropy. So I'm tempted to go back into the weeds on the past hypothesis definition, but I worry that we're getting too much into the weeds on that. There's a lot of non-weeds things that I want to get to. Well, I think there's a rather unfair thing I'm

doing here, which is this is actually a place where we have some minor but quite interesting intellectual differences. And I'm pushing my side against your position as a podcast host who can't kind of fire back with the full force. You totally are. And I'm not even sure whether I disagree with you or not. So we should have an offline discussion and then we'll put it somewhere on the internet to reward. We'll make it as a reward for the Patreon subscribers or something like that. But, okay, but you said something

that I think is very important to this bigger picture discussion. As the universe expands, there's this competition between things trying to equilibrate, photons and electrons and protons and helium, versus the fact the universe is expanding. And so it doesn't stay in equilibrium

because otherwise, like you said, the hydrogen would just keep fusing. the helium would keep fusing, you would all create all the elements up to iron, and then you would stop there. So the sort of hidden fact that is relevant there is that even though the second law says entropy increases, there's no law that says entropy increases as fast as possible, right? I mean, sometimes people try to invoke laws like that in living systems or dissipative systems or something like that. I mean, just very, very big picture. How far do you think we can go beyond the second law to be quantitative? about the ways in which entropy increases.

Yeah. So I'm not sure, and I'm not even sure entropy is always the reliable language to use here. I mean, we have, so one of the things I tend to do as a, as a philosopher physics, is try sometimes to step down a little bit from the big picture of how this could possibly work and say, well, what are the ways in which is actually being done and what are the assumptions that are going into them? So we've got a very developed theory of non-equilibrium statistical mechanics in the jargon, of the physics of systems that aren't in equilibrium

and are genuinely changing at time, but are big enough that you need to use these kind of averaging methods of statistical mechanics of thermodynamics to study them. And what you find in those stories is that entropy mostly seems to be a bookkeeping device. You can tell reliable stories about irreversibility and normally those are processes

in which the entropy of the system is going up and when they're not, it's only because there's some environment that's absorbing the entropy cost. But the actual the dynamic, it's not like, well, in the language I was using earlier about robust autonomous structures, we don't there seem to have a robust autonomous dynamics of entropy. You need to know more about the system than just its entropy to know what it's going to do.

And different systems do radically different things. And there doesn't seem to be any general law that systems want to really go to equilibrium at all. I mean, here's a sort of an example that it's at an abstract level has just the same structure as the hydrogen helium example, but it's perhaps easier for people to grasp. If I take a box and I fill it with a mixture of hydrogen gas and oxygen gas, or a glass jar or something, and I leave that on my shelf, it can sit there for years and be perfectly happily sitting there. And if you think equilibrium is the state that things make their way too fairly quickly and then stay there, you might be forgiven for thinking that that box of a mixture of hydrogen and oxygen is at equilibrium. But if you strike a match, you'll find out.

really quickly realize it was not an equilibrium. It's much more entropically favourable, much more thermodynamically favorable for the hydrogen oxygen oxygen to react and form water and blow your lab up. But there just happens to be a dynamical bar to us getting. And it's not a mysterious bar. It's about the fact that the typical energies of, there's a certain amount of energy of which hydrogen oxygen need to crash together in order to turn to water.

And that typical energy is much higher than the typical energy of actual hydrogen oxygen in the gas and so typically they just bounce off each other and keep going. If you waited really, really long times, it would happen, but it's just not going to happen spontaneously in the time scale when you're just sitting in the room. So we understand, at some level, we understand

the physics of that fine, but the point is it's that specifics of the physics that tells us that this system does not have increasing entry. So there can't, I take from examples of that kind, that there can't be, there can't be uniform or universal principles about how quickly

entropy increases or something. That's not to say that there might not be powerful gembalizations that are not universal, but nonetheless have a universality in physics jargon apply to a very large class of systems. As far as I can see, we don't very systematically have things of that kind so far, but that doesn't tell us anything about whether we could have things of that kind. So I don't know. I mean, this is more that kind of, that kind of. what are the most promising routes to develop

and exciting new piece of physics is a place where I have to some extent of now outside my comfort zone, so I'm not sure. There are some things you just can't do from the comfort of home. You can't go on a hike, you can't go see a concert, but what you can do is exercise, and with Peloton, you'll have a workout experience like no other without ever leaving home.

One of the great things about Peloton is the feeling that you're in a community, that there are other people participating, struggling, and sweating along with you. So even though you have the convenience of being at your own home, you get the common feeling that everyone is in this together. From keeping up with close friends to enjoying world-class workouts with people around the globe, you'll never work out alone. You can hand out high-fives on the leaderboard and join groups like Working Moms of Peloton or Tabata for Tacos. So with the Peloton bike, there's nothing like working out from home. Learn more at OnePeloton.com.

New members can try Peloton classes free for 30 days at OnePeloton.com slash app. Terms apply. That's O-N-E-P-E-L-O-T-O-N dot com. It's an excellent point to make that, you know, okay, there's the second law and entropy goes up, but that doesn't mean that entropy is necessarily the best thing to be keeping track of when you're trying to formulate these rules. And in fact, I would spin it in a positive way. is saying, as a researcher, like, there's a lot of landscape out there for potential discoveries

of better ways to state these universal rules that we don't have. Non-equilibrium changes in statistical mechanics is a rapidly growing field right now and sort of has been sadly neglected in the previous century. So I'm optimistic about that. Well, good. So this is a good sort of segue then into these remaking the connections we started with to other arrows of time. So we have a complicated universe full of all sorts of different things with manifest manifestations, I should say, of time asymmetry, which we're going to say are ultimately due to

let's say, let's call it the thermodynamic arrow of time. You know, the fact that entropy is increasing however you want to characterize that arrow of time. So why does the fact that ice cubes melt and don't unmelt help me understand why I remember the past but not the future or why I get older as time goes on. Yeah. I'm not sure I'd know the right Gemmell thing to say here. And I think a lot of the attempts to say it have never been, I've never found completely compelling.

I mean, look, here's a starting point that I think matters. When we talk about the way in which the physics distinguishes the past from the future, sometimes we characterize that as just there isn't a symmetry that reflects, past and future. But that I think doesn't fully get at it. I mean, there are, as you know, there are, there are microscopic bits of physics that in a certain sense genuinely break the symmetry of past and future, the decay of neutrons in a certain way or the certain, rather more esoteric than that, I guess, actually for that symmetry. Esoteric bits of particle physics actually seem to break the past future symmetry, but not in the way that matters from this point

of view, not in my, if you ran the video backwards, would it be manifestly obvious that you were doing it way? What, what I think matters? in the way these dynamics run differently falling backwards, is they have irreversibility and they have noise. Irreversibility, meaning you lose information. Lots of different initial states come together and end up in the same final state. Noise in the sense that it becomes, at least de facto,

impossible to predict which of many future states the system is going to make its way into. and those features of irreversibility and noise wash out information in the evolution process. So you can sort of see this if you look at predicting forward and backwards. If I've got something like the solar system physics, like I was saying for eclipses, the way I go about predicting what the solar system will be like in a thousand years time, it's exactly the same as the way in which we predict the well, retro-dict to use the jargon the way the world was a thousand years ago.

And we say this funny neologism retradict in this situation because we really are just doing a prediction just with time sets as negative. You can't, mostly you can't retradict macroscopic laws. If I've got the ice cubes in isolation, I've got my cup of, with a little bit of ice and there's cold water around it, I can predict pretty reliably the rate of which

that ice cube will melt in the length of time at which the ice cube has gone. entirely. I can't retradict that way. I can't work out whether actually the ice cube, which ice cubes were there before, how they were put in. That information is just lost. Maybe the microphysics still knows it, but at the level of description we're talking about, it's just gone. So I think at some level, these kind of ways in which our dynamics are irreversible, information losing, are also in the business of distributing stuff across space in a way and not bringing them back in.

These are the kind of features of dynamics that then suggest that if you try to build things in that dynamics, if you try to do information processing in that dynamics, that information processing is also going to have to have a clear distinction in the way it treats the past and treats the future. So that's the proof of concept. I don't think I've ever worked out fully to my satisfaction how the details of that are going to go.

Why does that general asymmetry of information processing specifically lead to the fact that we're putting down records about the past and making partially reliable predictions about the future or not vice versa? I think the quick attempts to do it are not convincing. Okay. The convincing attempts aren't fully there. I mean, there seems to be at least a naive tension between what you said, which is certainly true about the fact that there's dissipation and we lose information in some real sense with the fact that as a real purpose, I think that I'm learning things as time goes on. And I'm, I am writing books and there's information in the books. And so there's some local increase in the amount that I know about the past. And that's clearly going to be a little bit tricky to reconcile. Yeah. And look, I mean,

I guess you know, of course, by the same token, there's an apparent tension. It's long been recognized. Some people who shouldn't know better have used it to argue against science or for all sorts of excuses. It's long been the case that there's an apparent tension between the fact that entropy always seems to go up and the fact that we go towards equilibrium and the fact that I personally am not at equilibrium and don't expect you at equilibrium for some decades. And of course, the answer to that is unbust furious. I'm not a closed system. I can maintain my own low entropy by dumping entropy into the world around me, just like my fridge can stay cold by warming up the room. And at some level, these kind of information processing things have the same answer,

I think. You can, even if the physical processes are going on, on a systematically, in some says, losing information, then I could, provided I don't mind dumping the world into randomized states, I can still maintain an increasing information about the things that interest me. Which brings up a related worry that people often try to pinpoint me with. you know, if you think that all of the differences between past and future ultimately come from this thermodynamic gradient, if someone puts you in a refrigerator and your entropy starts going down, do you suddenly remember the future but not the past? I mean, is it possible to sort of completely reverse the arrow of time locally in a way that would make physics seem very weird?

Yeah. Well, not by putting you in the refrigerator. I mean, well, it kind of goes a bit to what we were saying earlier, about entropy not necessarily being the sole measure of irreversibility. Yeah. I mean, there are a ton of irreversible processes in your body, and even if you're cooling down so that your overall entropy is decreasing, those processes are still happening irreversibly. The dynamics that's governing all of the kind of biology in your cells is still happening in a way that draws a clear past future distinction.

Now, if you did something to you that systematically reversed those directions, that's another matter. but now you won't do that by the fridge. Now we need kind of space aliens and godlike science to do it. And we kind of know what would happen if you would run backwards. You'd, and if your environment was preserved, so if your environment was preserved to be compatible with it, then obviously it would just be like, you know,

leaving aside, I think, a slightly badly phrased quest about what they'll be like subjectively, then from the point of view of the person watching, you would just do everything backwards. But that's, I mean, obviously we set the problem up that way. That's trivial. if your environment is not also running backwards, I think extremely quickly the environment would mess up,

would mess with the way you were trying to run backwards and start you're running forwards again. Yeah, so this crude kind of thing, like just lowering your entropy overall, is not nearly enough to truly reverse your personal arrow of time. Exactly. If what we're really after here is dynamically reversibility, then the dynamically reversible processes are not changed,

by this. In entropy terms, all of those processes are still generating their entropy. And we're also thacking a big chunk of entropy out of your system just by moving bits of your body that are locally at equilibrium to bits of your body that are at local equilibrium at a lower temperature. and that overall entry budget, which is a sum of the various non-equilibrium bits of your body doing non-equilibrium stuff that increases entropy, plus the, the, the, the, the, the, the, the, the, the, the, the, the, the, the, the, the, the, the, the, the, the, the, the, the, the, the, it's going to be positive, it's going to be negative so that your entropy drops, but it's still the case that dynamics in your body is cleanly distinguishing between past and future. It's just that that crude indicator of which ways the entropy gradient going, uh, is, is, is, is, is a case.

doing it for you. There's more to thermodynamics reversibility than just the entropy gradient. Well, you know, I believe everything you just said, but I think you're missing a big opportunity for a startup that would not only preserve you in time, but actually make you younger just by putting you in a refrigerator. Like if you could develop some theory of that, you could be making Boku bucks, I'm just saying. If it worked, I think I could make a lot of money. If I could convince people it would work, I could be making your business match money, but I'm a little bit too honest. Yeah, the convincing people works is an easier task in making it work. So I think That's the whole secret to being in this business.

But okay, all right, with good. With all that wonderful stuff on the table, there is a looming specter over this whole discussion, which is that we've been speaking pretty classically, right? Good old, we even mentioned Isaac Newton and Maxwell and people like that. And there's a famous part of physics where it does not seem manifestly time symmetric, which is the active observation and the collapse of the wave function in quantum mechanics. So you know something about quantum mechanics. How do you think about, I mean, either generally or you're very happy to dive right into Everett in many worlds if you want,

but how do you think about the time asymmetry of measurement in quantum theory? Good. Okay, so my starting point is going to be that same kind of dynamics first, broadly reductionist starting point, that I think we just sort of took as a common starting point an hour ago. And that's going to be, so measurement here is just a special case. of large-scale interaction. Measurements happen when some relatively small-scale thing gets magnified up in a highly redundant way.

So the processes of measurement from that point of view are going to have to be special cases of this process of being magnified to the macroscopic. But of course, famously in quantum mechanics, that process is problematic. The process of measurement of maxcopic amplification day facto looks something like the randomized collapse of the wave function. As a practical matter, it looks as if what was originally a superposition of lots of possible

goings on snaps into being one of them. What's really going on to justify that is another matter, but that's kind of how it just looks from the point of view of doing the practical physics. The emergent level description is a description where I move from a superposition to a randomly selected element of the superposition. That process is blatantly manifestly time irreversible. And it's always amazed me. Actually, it's less true that it used to be,

but for long time, it's always amazed me that that kind of quantum mechanical arrow of time was the sort of, I don't know, the forgotten stepchild of the arrows of time in physics. It got very little air time compared to the other sexier errors of time, as you like. But really, it's, Because you say quantum mechanics is kind of fundamental.

It really ought to kind of matter here that it's turning up. And I think the reason why it doesn't, well, there's a claimed reason why people don't worry about these quantum effects. And there's a real reason, I think. The claimed reason is something like there'll be a standard line you'll get at the beginning of almost any sort of textbook of philosophy of statistical mechanics. It'll say something like, yes, I'm using classical mechanics. Yes, the world's really quantum. Yes, we better allow for the quantum effects. But quantum mechanics is technically, the quantum statistical mechanics is technically more complicated than classical statistical mechanics and the same foundation issues arise.

I've said those words. That is, I think, just almost exactly wrong as a statement of what's going on. I think actually for foundational purposes, I mean, I won't go to the details because it's a bit into the weeds. For foundational process, quantum mechanics is easier. There's a bunch of pathological features of classical mechanics that are not present in quantum mechanics. that I mean, if you want to calculate, then sure, heaven help you. But if you want to just work out the abstract structure of the problem, it is easier to be quantum. And the issues change profoundly.

For a bunch of reasons, and I'm sure we'll come onto more of them, but in the first instance, just precisely because of this clear asymmetry of measurement of macroscopic amplification. I think the real reason people who write textbooks on this stuff or monographs on this stuff do classical statistical mechanics, they're scared of the quantum. a measurement problem. And they don't want their beautiful pieces about statistical mechanics to be adulterated by all of this quantum mechanical stuff. I mean, I would agree with that if we interpreted the word scared as having a concern that it would derail the entire discussion. That's fair. Yeah. And I think the problem is it does derail the entire discussion. But that's another way of saying

that quantum mechanics is really important in understanding the statistical mechanical hour of time. But I suppose put it this way, look at that asymmetry we're talking about. That asymmetry cannot cleanly re-distinguished from the statistical mechanical asymmetry. It's not exactly the same as it, but it's intimately related to it. If you, I mean, there's a bunch of different ways of seeing it, but if you think that, for instance, almost every, if you think the collapse of the wave function is a genuinely random process at a really deep level, for instance, it more or less it more or less destroys any concerns that I can run my probability argument just as well forward and backwards. If it's extremely probable that the current state will evolve into higher entropy state,

but there are certain super special ways that the current state could be that could have evolved into a low entropy state. Well, even if you put the system in one of these super special states, it's not going to stay there for long because quantum randomness will just kick it around a bit. So if you had fundamental collapse of the wave function, you would break the asymmetry in time of the dynamics in a way that it's puristically really plausible would sort of out for statistical mechanics. Now, if I thought collapse of the way function was a good idea, then I could go on at some leg for how we could reform statistical mechanics things. I think for reasons that I think you share that this is a terrible idea. It's a good idea. It's a fundamentally wrong idea. It's worth exploring, but it doesn't work. It's not the right way to go. The right way to go,

I think, is something like the many worlds interpretation. We want to say that quantum mechanics really maintains all those superpositions. And the fact that it looks as if, we've got one of the possible outcomes at all there at once, rather than all of them at the same time, is perfectival. It's the, we just happen to be the kind of physical system that's sitting in one of those terms.

There's a David in one of the other branches after a quantum measurement who's just as committed to being David as I'm committed. And of course, there's a whole panoply of issues associated to many worlds quantum mechanics anyway, but to stick with the statistical mechanical issues, many worlds quantum mechanics is has time inversely variant because it's just unitary

churning equation quantum mechanics it's reversible in the previous senses and so there isn't any prospect in many worlds quantum mechanics of having some prior quantum error of time that underpins the the classic the statistical mechanical hour of time in a sense it's almost the other way around if you look at the kind of mathematics that's used to describe measurement and the amplification of superposition to the macroscopic and the thing that quantum physicists called decaherence, where it becomes impossible in practice to detect the many world's nature of systems because they get so tangled up with their environment.

If you look at the actual mathematics of all of that, it's all statistical mechanical. The kind of assumptions made to derive those equations are just the same assumptions as we use to derive the equations in non-equilibrium statistical mechanics. In fact, sometimes they're literally the same equations. There's a thing that people in decoherence theory and foundations of quantum mechanics use, you know, the decaherence master equation. Standard models of how the environment affects the model system.

This is literally the same as a standard equation that's used in non-equilibries, dismalionics and has been used from the 60s and 70s. There's nothing untoward there. No one's denying that, but it's surprisingly often not recognized. because the people working on one side of that divide aren't super familiar of work on the other side. I met that equation as a grad student interested in quantum mechanics in the 90s in the DG Heroes literature. It was 20 years before I rediscovered in that equilibrium of statistical mechanics. And okay, I'm a philosopher and I get to do other stuff. But even so.

Well, yeah, so I think we can take on board the idea that our listeners know that there's something called quantum mechanics and there's a wave function as a superposition of different things. And even that they know that you and I are both fans of the many worlds interpretation, which says that, like, you say, collapses aren't real. There's just this smooth, reversible evolution of the wave function. Okay, but then there's branching, right? And I think that's exactly where the time asymmetry comes in. But maybe before the time asymmetry, we should ask,

because this is a crucial question. And I try my best to give good answers to it, but it doesn't always satisfy people. What do you mean by branching? When does that happen? How often does it happen? How should I think about that? Good, good.

Okay. So to back up a little bit, here's how I think about it. If people listen carefully, I'm going to smuggle in a whole bunch of assumptions about the direction of time that we'll need to come back to. But ignoring that for the moment.

So what's going on when we measure a system or we allow a microscopic system to magnify up to the macroscopic is effectively that lots and lots and lots of other bits of the world get tangled up, get quantum entangled with the system. So the kind of full experiment I always start with. I've got something like Shreddinger's Cat, and Shreddinger's Cat is a light and dead at the same time. And the normal form of the quantum measurement problem says

that can't be the way things are, because when I look at cats, I never see them alive and dead at the same time. And somewhere in the background there is the idea that what it would be like to see a cat alive and dead at the same time is kind of like what it was like in the before days before the pandemic, where you just got really drunk with friends and you were so drunk you saw double. Yeah. That's something I'm seeing a cat alive and dead at the same time would be like having an experience of seeing a really weird indefinite a cat. That's intuitively what it would be like to see a cat that's alive and dead at the same time.

Intuition's a really bad way to work out how things look at the physical theory. if you ask the physics what it would look like, well, if I look at a cat that's alive, that's just a physical process. So my brain goes into some state or my body goes to some state you might describe as a seeing a live cat state. And if I see a dead cat, then I go into a state that you might describe as seeing a dead cat state. So what it's, the shoning equation is linear. So what that means is if I, never mind the details of that, it means if I, if I look at a cat that's alive and dead at the same time, then my state after that is, is, is linear. It means, if I mean, if I mean, if I look at a cat that's alive and dead at the same time, then my state after that, is, it. is seeing a live cat and seeing a dead cat at the same time. I'm not myself in some definite state of seeing a weird and definite cat. I'm myself in an indefinite state between two rather mundane cat seeing experiences. And the cat's connected up. So me and the cat together are jointly in a state of cat alive, I see live cat, plus cat dead dead cat.

And if I then tell you on the podcast whether the cat's alive or not, then you end up in a being told it's alive and being told it's dead state at the same time. and your listeners when it's played, I mean, much sooner than this really, but in fantasy for the moment, your listeners when it's played are in a being told by Sean the cat's alive,

plus being told Sean by the cat's dead state at the same time. And again, it's all correlated. So the whole quantum state of me and you and the cat and your listeners is a sum of two things, both of which are ordinary. One of them is the cat's alive. I told you the cat's alive and you broadcast on the internet. And the other one is the cat's dead.

I told you the cat's dead. And the broadcast that onwards now. And the rest of the world, as they hear about and find out for getting entangled, although the reality actually is it's faster and less picturesque than that. The ordinary kind of electromagnetic disruptions and things for me doing this will spread out at the speed of light and put you in a state of at the same time being correlated with David seeing a live cat and being correlated with David seeing dead cat, even before you're consciously aware of the answer to that.

So the earth as a whole, the solar system as a whole pretty soon, is at the same time, cat alive and catalide goings on and cat dead, cat dead goings on. And each of those terms separately is not interacting with the other one in any meaningful sense. They're not, the way in which quantum superpositions, quantum two things at once show up as two things at once is through interference effects that we can do between the two, which, where, where by, somehow things can reinforce or cancel out. So somehow the live cat bits could cancel out and the new cat bits could reinforce. That can't happen here. Once systems get big enough,

this is more thermodynamics reversibility actually, but once systems get big enough, you just can't do those kind of interference. There were too many moving parts to mesh up like that. So what you've really got is two chunks of physical reality. Each one is not interacting, with the other one and each one looks like

structurally is isomorphic to has the same shape as what things would be like if an ordinary definite classical thing was going on. So the world is two ordinary classical definite things going on at the same time.

And we need, or the universe is, and we need a term for some big chunk of reality that's doing its own thing and not really interacting with other big chunks of reality. World and branch are quite good terms for that.

Yeah. But they're emergent. I mean, my book on this is the emerging, multiverse. It's not the sort of fundamental at the deepest level of reality multiverse. All of this kind of multiple things happen at the same time is just more of that higher level autonomous going on that we were talking about earlier in the podcast.

It's just there's more than one bit of it at a time. Right. And actually it's worth saying for potential skeptics in the audience, if you signed on to the previous discussion about these emergent structures and how they could have autonomous laws, then you're most of the way to signing on to the many worlds interpretation of quantum mechanics. It's a very similar thing. Yeah, I mean, this is one of my original arguments for many worlds in philosophy, like way back when I was kind of making that jump from being a physics graduate student,

that once you accepted the basic rules of how emergence worked in physics, and you applied them to unitary, unmodified quantum mechanics, the many worlds just more or less happened. They weren't something you put into the theory. They were just given to you structurally, by the way it was set up. And so conversely, if you want to reject the ever-interpretation, you're in some danger of rejecting rather general rules about how theories at different levels are related

and rules that we use pretty much all over science and rules such that if we didn't use them, a lot of science would just stop working. But now I get to ask you like all of these tricky, annoying questions because you helped yourself to a very simple example of Furdinger's cat, which by the way, fans of the Mindscape podcast can buy a T-shirt with Ferdinger's cat on it,

at the Mindscape merch store, although my cat is awake and asleep versus alive in a dead, because I don't like to kill the cat. Well, you have cats, Dave. Exactly, yes. There's room for cat people. But when the observational outcome

is not something that only has two possible outcomes, but is continuous, like the position of a particle, is branching happening all the time? And can we even count the number of branches? Yeah. I don't think we can count them, or at least we can't count them none arbitrarily. I mean, look, the cat is an example of this, really.

I'm going to stick with the brutal version of the original Sherninger's cat, apologies to cat lovers. I mean, there are many ways in which that cyanide capsule can break. And come to that, there are many times of which the event can happen. The way people tend to tell Scherninger's cat in physics classes these days is you make a measurement of some particle, and according to the outcome the cat lives or dies, it's very clean. very discreet, but Schroding's original one had a radio isotope. Yeah. So we did actually get a discrete breaking into live cat and dead cat.

We got a live cat plus a kind of continuum worth of dying cats who die at different times. So how do you analyze that in the kind of emergent story I was telling? Well, you can't literally treat us a continuum. If you want to say that, you know, take two, I want to separate the various, dead cat states interstates that differ by, you know, 10 to the minus 25 seconds. Yeah. So that wouldn't work. There's on those timescales, the kind of quantum coherence of the radioactive decay matters.

I can't, uh, I, the, the, the degree to which, um, you know, the wavelength of the decayed particles requires constructive interference to make sense of it. It means if you, if you try to break the world up onto worlds that are that finely great, then my claim earlier on that there's no interaction between these worlds, no interference effects between stops being true. At that grain of analysis, there's interference. So I can't think of the cap state as being decomposed into branches that differ by 10 to the minus 25 seconds.

I slightly pulled that number out of the air. I wouldn't swear to the physics. There's a time scale such that that's true. Let's call it 10 to the minus 25 seconds. if I decide to do it on 10 to the minus 6 seconds, it's fine. The relevant coherence times for a radioactive decay are much shorter than that. So I can definitely regard the cat status as a superposition of hundreds of millions of dead cat branches differing by 10 to the minus 6 seconds. Can I get it down to 10 to the minus 12 seconds?

Yeah, probably. Can I get 10 to minus 18 seconds? I'm not sure. And there's no exactly sharp line where that stops working. There'll come a point when interference becomes such that the description in terms of branches becomes unuseful. There's, that's the point to which the kind of, there won't be an autonomous high level dynamics, that's description. And that's the point to which the kind of real pattern stuff that you're attributed, that dad dead it talks about, the Utukk had earlier, that that's sort of my gemal license to say, I've got higher level going on. that license is revoked at the point of which you get to that that finer grade.

But on a course a grain than that, everything's fine. And there'll be intermediate points where you can kind of get away with it, but it's a bit fuzzy than you might like. And so depending on your tolerance for error and the degree of precision in which you care, you're going to reach different conclusions as to exactly how finally you want to break things up. But the basic claim that there's a vast number of different cats is robust against those various ways of preciseifying it. And there's a lesson or a philosophy implicit here. Maybe you made it explicit.

But look, these branches are just kind of approximations. They're useful ways of talking about things, and you can't demand too much precision of them, just like you can't demand too much precision of treating the gas in a box as a fluid rather than a bunch of atoms. That's exactly right. I mean, this is, again, this general lesson about emergence being brought into quantum mechanics. Um, it's just systematically true that emergent accounts have fuzz and slightly blurry boundaries and no really sharp exact rules as to, um, where you divide them out. That's nothing to do with quantum mechanics. That's just generally how the world is structured. I mean, there's a kind of, um, there's a kind of temptation in talking about quantum mechanics and talking about many worlds in particular. It's less now, perhaps than it was, but it's still around to say that, uh, there's two options. Either something needs to be exactly and precisely.

find in my microscopic physics, or it needs to be an illusion. But hardly anything in the world fits those categories. You know, you and I are not exactly and precise defined. There is no Sean David interaction term in the Lagrange's of the standard model. And yet we're not illusory, or at least if we are, then the word illusion becomes, loses its value here. Everything higher level has this fussiness to it. And again, the many world's branches, just, have that fuzziness in the same kind of way. But the things that are still true, no matter how much you sort of tweak the setup, those are the robustly true that Scherninger's cat splits into a vast number of branches.

It is not very vastly true that the exact number of those branches is 10 to the power of 25 or 10 to the power of 18. It's robustly true that there are at least hundreds of millions of them. It's robustly true that there aren't 10 to the power of 100 of them. But there's no robust answer to the middle. Now, I should confess, I mean, various of my more, of my sort of more purest metaphysically inclined, um,

uh, colleagues are tearing the hair at this point, because the metaphysics here is chaos. Um, but I think, I, I think our metaphysics has to be responsive to our science. And so to some extent,

insofar as we need to change the way our metaphysics work to make sense of that kind of way of talking, that has to be changed in that way to do justice to science. And you, you seem to, have an implicit answer to this question and something you said earlier, but there's the question of when the branching happens,

how quickly does it spread? Should we think of the branching as being simultaneous in some arbitrarily chosen reference frame throughout the universe? Like when Alice and Bob are far apart with their entangled spins and they're going to measure them. When Alice measures hers,

does Bob split instantly? Or is there some speed of light delay there? Or is that a human construction and we can choose it either way? Yeah, I mean, it's kind of somewhere in between. And it's sort of trades on an ambiguity about what we mean by branches here that doesn't matter in small systems, but matters when you've got the Alice Bob situation. So here's something that doesn't depend

how you set it up. The quantum state of the region of space time containing Alice will not change until light has had a chance to cross the gap from the region of space time describing Bob. That's the fundamental lower, or more fundamental lower level description before we talk about branches. I mean, there's a bit of branching even to talk about a classical space timer Alice and Rob, but at the level we're thinking about, we're not, we're not thinking of branching here. If you describe in branching terms, the way I prefer to talk would be to say that I want to think my branch, my piece of emergent structure is something that's expanding on the forward

light cone. So Alice branches her future. The branching spreads out from Alice at the speed of the fastest interaction that's doing the the branching in practice that's always going to be light for big systems. Bob also has branching happening and eventually those light cones cross over and the Alice light cone. Sorry, I'm using my hands here as a podcast. The Alice light cone crosses Bob. The Bob light cone crosses Alice and then Alice and Bob split again. So Alice and Bob experienced two sequential splitting events, one when they do their own measurement and one where they come into the causal future of the other measurement. But sometimes it's convenient to talk more globally. If you want to write down a quantum state and say,

we'll break this state into Alice Bob both gets spin up plus Alice Bob both gets spin down, plus Alice gets spin up, Bob gets spin down, plus Bob gets spin up, Alice gets spin down. Then we'll want to use a global notion of branching. But at that point, we're no longer picking out a sort robust higher level feature anymore. We've added a piece of pure convention. And the piece of convention is picked up by exactly what you were saying earlier that we've chosen an arbitrary reference frame. So we could talk that way now if you want to,

but nothing physical is happening instantaneously, even at the emergent higher level sense. There is something happening at the immersion higher level sense, but only on the forward light cones of the measurement. At least this is the way I think about it. Yeah, no, I think it's appropriately, I think it is interesting because it is a kind of a pressure point of emergence and subjectivity and objectivity and all these things. So, you know, again, for the youngsters in the audience, there's work to be done in thinking about these things.

It's not all figured out yet. And we haven't, you know, to bring it all back and close the circle, we started this by saying that the quantum measurement process is a manifestly time irreversible process. So how do you explain that, Mr. Everett? Right. Well, I don't necessarily know the answer. I mean, this is, I'll give you a classic philosopher's way of solving a problem, which is by reducing it to a bigger unsolved problem. Okay.

So claim, this branching is a special case of thermodynamic irreversibility. That goes back partly to what I was saying earlier about the self-same equation is happening in a different context, but we can break it down. more physically. Here's the kind of argument you might give as to why Schrodinger's cat causes everything to branch out from it. I mean, you know, a random photon is coming in towards Schrodinger's cat. If the, it's coming in from outer space, let's say. If the cat is alive, the photo, then the cat's standing up, the photon bounces off the cat. If the cat's dead, the photon harmlessly past the cat and bounces off something different. And so now the quantum state before

the photon arrived was, as I suppose this is the very first photon, is cat alive plus cat dead, photon coming in. The quantum state afterwards is cat alive photon bounced off cat plus cat dead photon missed cat. And this is an entangled state. It's a joint state of both of them. The photon is now recording the quantum state of the cat. And then the idea is millions more photons come in and to a higher and higher degree, the cat is just thoroughly entangled of all these photons. Okay, but suppose we run that process backwards,

the backwards description of what's going on would be something like initially, the photon is entangled, quantum mechanically tangled up with the cat. And so the total quantum state is something like photon coming on a one trajectory dead cat plus photon coming on a different trajectory live cat. And if you evolved that through,

the photon coming in a one trajectory, in on the live cat trajectory bounces of the live cat, the photo coming in on the dead cat trajectory keeps going. And now I've got a single photon coming out no longer entangled with the cat. And if every single photon was coming in in quantum states like that, then at the end of all that process, the cat would be in its, the cat would have started in this ridiculously entangled stage. At the end of the process, the cat would have been completely pure. So why do we think one of those happens rather than the other happens? Well, in some sense, we think we think. think that that incredibly delicately entangled state of cat and on the million photons

is just a ridiculously implausible, ridiculously unlikely way for the system to have started off. Whereas the state with just all the photons coming in any which way, that was a perfectly natural, reasonable state for the system to have started in. But as I've kind of shown, that's an explicitly time directed assumption. The state that I've said is completely outrageous. It's just the state, it's just the reverse of the state the system will in fact get into in the future. The state I said was perfectly normal, was one that I would have got into if I ran the ridiculously outmajor state backwards. And all of this is exactly analogous to the likelihood of the atoms in a glass of water being ready to make an ice cube form spontaneously.

Absolutely. I'd say it's not even analogous. It's the same thing. It's just being applied to a different problem. I mean, there are some slight quantum subtleties, but basically it's the same thing. So the very same things that drive irreversibility in the ice cube drive the fact the system's branch rather than remarging. I mean, going back to the thing you said right at the beginning is, do I distinguish different hours of time? This is one of the reasons I don't really. If you start talking about the quantum era of time and the statistical mechanical hour of time, you kid yourself to thinking these are different things. Well, to non-everidians, they might be different things, right? That's fair. That's fair. Although it also shows why Everett is... Better.

It shows why there are awkwardnesses in not being ever etienne. Let me put it that way. Because these things about the way systems naturally evolve and decahira are happening anyway. It's kind of weird if separately from that, you've got an entirely different mechanism that really imposes the direction. I mean, if you have some different explanation of the arrow here, you have to ask, how does that mesh with the decaherent story? Because it's going to happen anyway. It's empirically tested. Not for cats, but for mesoscopic systems.

So just to restate that, the thing about Everett, whether it's the fact that there is branching or it fits into this bigger statistical mechanical picture, is that all of the processes you need to make Everett work will also happen in everyone else's version of quantum mechanics or statistical mechanics or whatever. But then they're saying, and I have extra processes also because it makes me feel better somehow about the explanation.

Yeah, I mean, yeah, I could make. the case for defense against that. But yeah, that's the basic shape. We're among friends here. But okay, so, and I'll get back to a difficult question then. How do you think about the past hypothesis in these quantum mechanical terms? Is it just one past hypothesis for the early universe and its wave function? Or do we have to be more subtle about different senses in which the early universe had low entropy?

Well, this goes back to why I would rather not place the past. hypothesis is a statement about low entropy. I would rather put it to a statement about not having lots of delicate correlations entangling. I think the past hypothesis is a kind of cosmic version of the hypothesis that all the incoming photons were not complicatedly entangled. If our past hypothesis that the initial quantum state was a relatively simple state without a bunch of complicated entanglements, that's probably mostly what we need. Good. So actually that's, in fact, that's perfect because we can wrap up the conversation by going back to the weeds that we were in a little bit ago that I said, let's defer that. I think we can do it now because let me try to

say, if I understand what you're saying in slightly different words, then you can tell me whether I've gotten it completely wrong or not. There's a version of the past hypothesis that just says the early universe had low entropy. Oh, and by the way, it was in a low entropy state that within its macro state was pretty generic, right? It wasn't not, it wasn't just low entropy. It was low entropy without any secret conspiracies that was going to make it even lower entropy, right? That's one version of the past hypothesis. And there's another version that says,

look, we look and we see the cosmic microwave background in our telescopes. It's true that if the question you asked was, of all the ways you could have seen that, what do most of them look like? And of all the ways the early universe could have been to give us this image in our telescopes, what do most of them look like?

and most of them look wildly and homogeneous with crazy fluctuations and, you know, gravitational differences from here to there. But there were, there were, there was structure in the specific microstate that canceled all that out so that we see smooth radiation in our telescopes.

And, yeah. And your point, if I understand it, is, sure, if the question you're asking is just what do most such early universe states look like conditioned on what we see, that's the answer you get. But if you add to that, a requirement that what do most early universe states that didn't have crazy correlations in them

and conspiracies look like that would give us our state today, then you automatically can derive the fact that, oh, it had to be pretty low entropy in our coarse grain sense. Was I close there? Yeah, that sounds right. Put it this way. If the early universe was in a non-crazy microscopic state, then something like the second law holds and the macroscopic dynamics for that universe, have entropy going up.

Yeah. And once you've got the macroscopic dynamics have entropy going up, then obviously the entropy of the early universe has to be learned and the end to be the present universe.

Good, yeah. So I think that I have not really sat and chewed on that different formulation much myself, but I'm not objecting to it. I'm not philosophically opposed to it. I think it makes perfect sense.

And in fact, it leads to, I already said this is the final thing, but we have another final thing now. Sure. It leads into this the questions that arise in the other. direction of time. We've been talking about the past, but we also have the future. There are some

of my best friends who worry about the fact that in the future, we have a positive cosmological constant that is making the universe accelerate. Everything will eventually empty out, and we're in what looks like equilibrium in some sense, just empty space. Desider space is the technical term. But DeCidder space has a non-zero temperature, according to Stephen Hawking and Gary Gibbons in the 1970s. So there could be random fluctuations. And if you wait a very, very long time, you will get fluctuations into Boltzman brains

or even entire Boltzman galaxies or universes and so forth. So are we worried about getting back to the recurrence objection that Zermelo gave to Boltzman years ago where if you say the universe started 14 billion years ago, but lasts infinity years to the future, who cares about the first 14 billion years? most of the occurrences of people like us will be random fluctuations in the future. Is that something that you personally lose sleepover?

A lose sleepover states it, but I think it's a serious concern. I think it's a little subtle whether it's true, partly for reasons about what fluctuation really means in an everettian universe that you've talked about, partly because ultimately this is probably a physics that is tentative enough that we should be cautious about him. But yeah, I mean, notwithstanding all of that, there is something concerning about it, I think. If the universe is such that it predicts that, you know,

it's full of beings who have, probably fake histories, but entirely true histories that come from fluctuations of their own galaxy into existence or something, I think there's some reason to be worried that the theory is self-undemining. I mean, look, in a certain sense, it's, there's a bunch of problems, I think, that come up in physics where, um,

we, we use infinity pretty casually in physics and mostly we use infinity as a, a convenient approximation for large finite things. I mean, particle physicists talk about, uh, collisions between particles and then we see what happens at infinity. And some philosophers, you know,

better worry about exactly how you formulate those infinities. As my colleague Porter William says, the main tube in the LHC is like about three metres wide. So whatever at infinity is, it's much less than three meters. So in case like that, all we really mean is something like much bigger than the scales are interested. It's easier to approximate things by infinity than by some large finite number. Every now and then a real infinity turns up. And when our theory has real infinity, in this case, the literally infinite future of the Ducity universe, then I sort of think at some level, I'm not certain we know how to theorise about things of that kind. And that's not to say, therefore, we should be relaxed

about the Boltzmann Braids that you talk about. It's more the opposite to say this is a reminder to us that in positing something that's really infinite, whether it's infinite into the time like future or infinity to space, it's not that that posit's illegitimate, but that posit is a very substantial deep thing that goes beyond our mathematical use of infinity's other places, and we're right to kind of pause and worry about it? Yeah, I think that's actually very much in line with my own view on these things. Like, it's not a showstopper for anything. It's a reminder.

There are things we don't understand, and we shouldn't be too glib about them. Some of my colleagues in cosmology think that it's a reminder that you've lost your mind, that you worry about these things. So for the truly very final answer, are you? you mostly encouraged or mostly discouraged by the relationship between philosophy and physics these days? Do you think that there's been more overlap that's been constructive or do you spend more of your mental energies sort of frustrated that it's not even better? I'm mostly positive about it. I'm, I suggest that question reflects more people's natural disposition than their objective

judgment to the field. My feeling is the philosophy of physics and up at its game and has, or bits of it at least, have increasingly engaged with questions in physics that they genuinely bother physicists. And I think, firstly, physicists, it's not true that physicists are not bothered about conceptual problems. They're not bothered about conceptual problems that are seen as sterile. And I think the quantum measurement problem has traditionally often seem sterile to physicists. It doesn't seem to lead to something. But the, I mean, without having time to talk about what this is,

imagine your readers, your list has come across it. I mean, the black hole information loss paradox has had the, the finest minds in theoretical physics, tearing their hair out over it for the better part of 50 years. Yeah. And it's a conceptual problem. It's not a calculation issue. It's not that these things don't matter.

And I think if you're, if you can, if you can come to the table in that conversation, with something to contribute, then I think you can relatively easily get across the prejudice that can occur. I mean, I think you do run into genuine prejudices. I think it helps to say the word conceptual rather than philosophical. I think it helps have a physics PhD more because you can say, I have a physics PhD because of any concrete knowledge you got from it. But I think it can be done.

I think there are good conversations they have and a lot of people are willing to talk. And I think, I mean, because I've mostly put that in terms of like, what are the issues of engagement physicists, I think it is true that philosophy of physics has some bad habits of its own here that it can help get over. I think philosophy of physics has this slightly awkward situation where mainstream physicists are not naturally inclined to talk to them. But people in much more niche areas of physics who are struggling themselves to get physicists to talk to them are much more willing to talk to philosophy. philosophers. And so philosophy of physics to a degree that concerns me sometimes can often be a philosophy of dissident physics or minority physics, which isn't to dis any of that. I mean, we don't do science by majority vote. But it does often mean that the center of gravity and energy in philosophy physics often struggles to be on the conceptual questions that physicists really

care about. So, yeah, I mean, that's a kind of cautious optimism about it. I think there's still problems, I think there are institutional structural problems. And I also think bluntly, many people doing philosophy of physics or who want through philosophy of physics aren't necessarily always doing the homework to actually get on top of things to the degree that they don't make relatively elementary technical errors that will be seen correctly as invalidating what they're doing by a physicist they're engaging with. So ultimately doing interdisciplinary work is hard. You have to collaborate, you have to talk to other people, you have to put the time in. And there's always a prejudice on both sides against it that you have to deal with.

But yeah, I'm kind of optimistic. I think the best days are ahead of it for that point of you. You did a good job of being optimistic for them being very cautiously optimistic, but we will take that for what we're looking for here. So David Wallace, thanks very much for being on the Winescape podcast. Thanks for having me. What if you could have even more and more and more help to pursue your goals? At LPL Financial, we offer more ways for advisors and their clients to thrive.

So what if you could? Paid advertisement. Investing involves risk. potential asset principal LPL financial LLC member FINRA SIPC