Daniel and Kelly’s Extraordinary Universe - Could a grand unified theory of physics be impossible?

Episode Date: January 10, 2023

Daniel talks with Dr. Katie Robertson about whether there even is a single unified theory of physics for us to discover.See omnystudio.com/listener for privacy information.

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Listen to Brown Ambition on the IHeart Radio app, Apple Podcast, or wherever you get your podcast. The long arc of physics bends towards what exactly? Over the centuries, we have developed theories to explain. the universe and then seen them overturned replaced by something new. Einstein's gravity replaces Newton. Quantum mechanics upends a deterministic universe. It feels like progress, but is it? We imagine that there is a single, beautiful, simple set of laws that control how the universe works and that with each new idea we are getting closer to the deep truth. But what if there is no single deep truth? Then what is it that we are learning anyway?

Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I'm spending my life chasing the truth of the universe. And welcome to the podcast, Daniel and Jorge Explain the Universe, a production of IHeart Radio, in which we don't shy away from trying to discover the truth of the universe as blindly bright as it may be. We bring you as close as possible to our current understanding of what's out there in the universe, from particles to galaxies, what we do and do not understand. My friend and co-host Jorge is on a break, so I've invited a guest to chat with us today about this journey to understand the universe. I personally see our cosmos as if it was a giant detective novel. We gather clues and try to figure out what is going on. What's story out there explains everything that we see. As we collect more information, we can rule out

ideas that we had initially. Sometimes we get a clue that provides a huge plot twist. What? The universe is fundamentally random? What? Gravity isn't the force after all? Mind blown. Those are my favorite moments in science because they make me feel like we are taking steps towards the truth. Like we're We're on a road towards figuring out what the story of the universe is. Because any good detective novel has to meet just one simple requirement. It absolutely, positively, without any deviation, has to follow its own rules. No cheating, no magic, no sudden changing of the rules to reveal the murderer. If the reader is going to have a fair chance at puzzling out the answer, then the clues have to make sense.

They have to be real hints about the actual underlying story. There has to be a coherent story, one that's self-consistent. And we expect that to be true about the universe as well. Most of us who are not philosophers or super skeptics think that the universe is real, that it's out there, that it's following some set of laws, and that by paying attention, we could figure out what those laws are. We feel like we're chipping away at it from different angles, chemistry, biology, physics, but in the end, we are all working towards revealing a single larger

truth. We're each turning on our own lampposts and shining light on what's under them, with the idea that more light means a better view and that eventually we will be able to see the whole picture. But how do we know that's true? Is it possible that there isn't a single coherent story of the universe so that laws in different contexts and different situations could be incompatible with each other, that each field of science might be its own separate patch, not part of a larger quilt? So today on the podcast, we'll be asking the question. Could a grand unified theory of physics be impossible? To help me tackle this grandest of questions, I've invited a philosopher of physics who spends

all of her days thinking about this particular question. So it's my pleasure to welcome to the podcast, Dr. Katie Robertson. Katie has degrees in physics and philosophy, including a PhD in philosophy from Cambridge. She's now a fellow at the University of Birmingham, where she thinks about how the microscopic laws of physics weave themselves together to form the world we experience from thermodynamics to the arrow of time in black holes. Katie, welcome to the podcast, and thank you very much for joining us. Well, thank you so much for having me. So let's get started by getting to know you and your interest a little bit. Obviously, black holes and the deep mysteries of physics are fascinating, but I'm sure there

were many directions open to you from experimental physics to theoretical all the way to philosophy. What made you choose this path? Why do philosophy rather than theoretical physics or experimental physics? Well, I think I was always interested in the conceptual questions in physics. I remember getting really confused in high school physics and asking my teacher, like, oh, but what is an electron? He was like, well, that's kind of a philosophical.

philosophical question. And so then having studied physics and philosophy together, it was kind of like the philosophy of physics, which was the thing that really kind of grabbed me. And I was really bad at experiments. I managed in my first year labs to get the gravitational constant to be a thousand. So I think that was never going to be an avenue for me, unfortunately. What's 40 orders of magnitude between friends anyway? So do you take that's a philosophical question to be an encouraged? or like a discouragement from a physicist that might seem like, you know, that's not really territory we want you to be asking. But it sounds like you took it as like, yeah, go dig deeper into that. I guess it's a sociological thing, isn't it?

Whether you think it's a good question or not. I mean, I guess in some ways it's not an encouraging thing. Like, if you think something is purely philosophical, you might think that means that it's out of the reach of empirical support. And that's normally seen as a bad thing, right? The kind of key feature of science is that we can do experiments and get everything. evidence in that way. But I think there's a kind of like a continuum between the two, between physics and philosophy. And often in the history of physics, lots of physicists have had certain like philosophical convictions that have led them to their results. So I think it's quite interesting

seeing how the two kind of mixed together. So yeah, no, I find it quite interesting. But yeah, so some might find it discouraging to find out it's a philosophical question. I think that a lot of the questions we do in physics are philosophical. And a lot of physicists have strong philosophical positions, which is usually, I don't do philosophy, which is actually, of course, a strong philosophical position, right? Yeah, in itself. I mean, somebody like Einstein, right, with his worries about quantum mechanics, were really driven by, like, philosophical views of what the world shouldn't be like.

So, yeah, I guess it's one of those things, you know, you've got philosophical views. It's just whether you've explicitly stated them and come to terms with them or whether they're kind of hiding buried in you somewhere. So then of all the questions in physics and philosophy, what's the one that keeps you up at night? I'm often describing science to our listeners is like, just a bunch of people who are curious about the world, everybody's chosen their one question to devote their life to. So what's the question you would ask like the Oracle or super advanced aliens if you had the opportunity? So one question that I have, though, though maybe it's like one of those philosophical questions that doesn't have a clear empirical answer that maybe the aliens wouldn't be any better.

off with but one question I find really interesting is this question about what the relationship is between like what our theories tell us the world is like and what the world is like so is it going to be that our theories you know miss out some stuff or are we like using you know often we have like kind of extra mathematical structure more than what we need in our theories and sometimes we can know that we can know there's extra kind of descriptive fluff there and sometimes we don't so I I guess I'm interested like, you know, what we should read off from our theories, you know, which bits should we take

to be true and really about the world, which bits are kind of, just kind of extra stuff that doesn't really correspond to anything. Wonderful. Well, I love how philosophy lets us ask, like, profound questions about things that seem ordinary, right? Like, is our science teaching us anything at all or whatever? And one question that I really struggle with is like, why can I watch a ball fly through the air and describe it using fairly simple, equations. You know, why is it possible for me to do that? And the naive answer and maybe the listener out there is thinking, well, because the universe follows laws and we can deduce those laws, no big deal. But I think as a particle physicist that probably those laws, if they exist,

they operate at the microscopic level, right, on particles or strings or whatever the basic bits are. So if I'm watching those basic bits themselves, I can use those laws to describe them. But I'm not, right? I'm massively zoomed out. If I'm watching a baseball, it has like 10 to the 29 particles in it? Why, if I'm looking at 10 to 29 basic bits, do I see anything that makes sense? Why isn't it all just fuzz and chaos? I mean, I don't have a simple rule that predicts the role of a die or the movement of the stock market because it's so sensitive to those tiny details. Why isn't it always like that? Why, when you zoom out in the world, does any sort of simplicity seem to emerge? Can you help us get a grasp on that kind of question? Yeah, I mean, there's been

a lot of different responses that people have given. So, I mean, there's a kind of defeatist response, right? Which is, well, it's very complicated, but we've got to do what we can to make sense of it. And so even though it appears that things are simple, that's just how we have to approach it, you know, in the same way you might think, well, the reason that we have to, I don't know, use Newtonian mechanics or do biology and chemistry is just because we're really bad at solving the Schrodinger equation for complicated systems. And that's why we have these kind of other theories. So that's the kind of like defeatist option. which is that we're just not good enough at solving the really tough things.

If we could, maybe we'd use those instead to understand the ball going across the room. So is that saying that the universe really is complicated and that we're just making like a weak approximation of it by describing it simply? It's sort of like it's part of the fact that we're, you know, in the same way that if a child doesn't know very much language, then they're going to describe the world in a kind of much less colorful way perhaps than an adult would describe the world. and so that makes it seem like the reason we see any simplicity is like

well we're just a bit simple so we have to be able to see things that way which I find a bit well it's the fetus but also I think it'd be kind of amazing right if we were just not very good at describing the world and the ways in which we did describe the world were so successful right like it seems like it's not just

sometimes people have this distinction between the way the world really is versus like what's our perspective on it so you know we see the flower in the garden in a really different way from how bees see flowers in the garden. Because our eyes are sensitive to different parts of the electromagnetic spectrum. So you might think, well, maybe some of the simplicity is a bit like the kind of color of the flowers. It's just like how we see things rather than how they really are.

But I don't really like that way of thinking about it because I think we're getting something really right, you know. It's not just that we're using simple laws because we're simple people. It's that there is this kind of macroscopic simplicity out of this kind of microscopic, kind of microscopic kind of incredible complicated stuff going on nonetheless at this kind of higher or emergent level you get this kind of simplicity right like to use your analogy of a child maybe a child doesn't use flowery language but when they say me want candy you understand right it works it's successful and when i'm taking an approximation of something because i can't do the full calculation and taking the first second third order of perturbation theory i mostly get the answer right and i can ignore the other details and

if the universe was just chaos and fuzz then that wouldn't work right and it doesn't for example when I try to predict the stock market, trust me, I've tried, you know, it doesn't work. So I feel like the defeatist answer seems to totally fail at explaining why simplicity emerges. And also, do the defeatists call themselves the defeatists? I guess maybe the more correct way of labeling them, but it might be somebody who would say something like there's just a kind of methodological autonomy. Like the reason we have this methodology where there's all these different scientific disciplines that focus on different things, they have their own conferences, by and large,

they just talk to each other rather than, you know, sometimes talk between each other. But, you know, the reason we have science kind of like hived into these different kind of institutions is just because that's how we go about doing it. So you can think of that as being the kind of methodological autonomy. It's like, perhaps the answer isn't just that it's, you know, it's simpler to use Newtonian physics for calculating what's going to happen with the ball across the park. It's not just that. It's also that that's the kind of right laws to be using at.

that level. So that would be a kind of different way of thinking about it, where it's not just that we're not very good at solving the Australian equation for complicated systems. It's that actually these other laws and equations are kind of more suited in the same way that if I ask you what the weather's going to be tomorrow because I want to go to the park, the kind of right grain of answer is like, you know, it'll be sunny, it'll be raining. It won't be like to give me a kind of complete survey of what the weather across the whole world's going to be like. So, yeah, one alternative answer to just saying, well, we've got to do things that way in the kind of defeatist or kind of methodological approach would be to say, well, actually, there's

kind of different laws are appropriate for different things in the same way different tools are appropriate for different tasks or something like that. But does that reject like reductionism? Does that say that those laws, F equals MA, for example, doesn't arise somehow from like the towing and froing of the basic bits that they emerge at the. their own level? Or are you suggesting that they do emerge and that there just are naturally these different scales which the universe coalesces into simplicity? Right. So I think reductionism is very fraught debate because lots of people mean different

things by it, right? Like some people mean by it, again, a kind of methodology like, hey, you want to understand something, look at its component parts. That's the best way to go about doing it. If I want to understand this thing, I'm going to understand all of its parts and that's how I'll understand it. That's what a particle physicist would do, for example. And, you know, that also goes across other sciences. For instance, you know, if you want to understand disease, some people think really the crucial thing to understand is the kind of genetic factors that lead to that disease.

Other people might think, oh, perhaps there's a kind of the environment plays a large role. So this kind of theme of understanding things in terms of their parts kind of goes across all the different sciences, I think. That's one form of reductionism. Another form of reductionism would be to kind of be a kind of more meaty claim about the way the world is. So to say, really all that exists is the very fundamental particles.

Whatever they turn out to be in the end, they're going to be all that there really is. Everything else is just a kind of different way of talking about those things, a very complicated different way. But I think that probably the most useful way of talking about reductionism is to talk about how we understand, like, how one theory is related to another. So the kind of obvious example of this is, you know, Newtonian mechanics was very successful. ultimately we think that it's not quite right and if things are either very heavy or moving very fast or very small then it doesn't work

but we can show how in certain limits if your football is being kicked by a human rather than a kind of incredibly strong alien that it's going to be traveling at speeds when Newtonian mechanics are still really good so we can understand how those theories are related to each other in particular we can understand

how to construct one theory out of another you know in a particular limit you get back your one theory from another And I think that relationship is really useful for seeing which scales you think different theories will work at. Because if you can show that you're going to get back Newtonian mechanics in the low velocity limit, then that kind of explains why Newtonian mechanics was really good there. And I think that's a way of seeing that as a kind of one pattern emerging out of kind of another more fundamental pattern in a certain regime. So I think reduction is really helpful for understanding how the different kind of theories and laws that we have,

all fit together. You ask whether that's compatible with thinking there are laws at different levels. So some people have said, no, if you've got kind of emergent laws, emergence, you know, is one of those words that's like so controversial what you mean by it. But for some people, emergence just means the failure of reduction. So for those people, that story about getting theories back in different limits or whatever, that's going to be a case of reduction. And if there's reduction, then there's no emergence. So there aren't these kind of, there's a kind of no meaty sense in which there's these kind of new things at higher levels. Really, it's all

just the fundamental things, and you can show why you thought there were other things. You can show why you thought there were Newtonian forces. But really, there isn't. That was just the kind of old way of speaking. I kind of prefer the view where you think of different laws as emerging, and they're all kind of on a par with each other

in the sense of, like, some laws are more fundamental than other laws, but none of them are like kind of second grade citizens, you know? They're like, you get the laws with Newtonian mechanics emerging out of relativistic laws, but that, you know, that's just the right laws to have in that domain. When things are going nice and slow, then that's the way to, the kind of laws to use. I like your organization of the topics there in terms of complexity,

like the idea that maybe Einstein's view of gravity is more complete, but it's too complicated. Like if I wanted to solve the question of like, what is the Earth's orbit going to be? And you gave me Einstein's gravity. I'd be like, well, I'll be here. for a while, whereas Newtonian mechanics is going to give me the answer straight away, and it's also going to give me a story that I can tell that I understand. And maybe this is also the argument you're making that some of these laws are just more useful in their explanation. Like if I want to tell you, oh, what happened to the ball this afternoon? And then I give you a description of all 10 to the 29 particles and what each of them did doesn't really answer your question. But if I say, oh, it flew in a

parabola, it landed 480 feet from home base or whatever, that's sort of the story, the explanation that we're looking for. So does that mean that it sort of depends on the question we're asking? That there are no more fundamental rules. They're just sort of like laws that answer the questions we're asking. So I guess the worry with that is that if it just depends on what question we're asking, you might then think, well, that just depends on what you care about. And so it's really just tied into your interests. And then that kind of starts to look like it's dragging us in the defeatist direction where it's all connected to what we understand about the world rather than how the world really is. So I think that a helpful way to go is

to think of, in what sense is Newtonian mechanics better for describing the trajectory of the ball? And I think that the kind of right answer for that is, well, when somebody asks your question, you need to give them the right amount of details. It's not just that that's more useful, like that's the better explanation. And so that would then mean the kind of structures and laws associated to the kind of less fundamental theory are doing the kind of best explanation. And normally people think if something's giving you the best explanation, that's the thing we should take to be true. So this is sometimes called like inference to the best explanation. What's the reason why the apple fell to the ground? Is it because the fairies pushed it or is it

because Newtonium mechanics, or is it because I looked the wrong way? You know, you can think of all the different possible explanations and the kind of best explanation is the one that we normally take to be true. So if you can kind of give a reason why these non-fundamental theories, you know, we think they're less complete, they're missing some of the details about the world, but nonetheless we think that they're perhaps giving the best explanations, then we can still be committed to all of this kind of emergent structure

and we don't have to relegate it to just kind of useful stories that we tell, we can really say that it's getting at what the world's like. We're lucky that there's some simplicity, and that's kind of useful for us. We want to say, and that's really this interesting fact about the world. This is a kind of deep thing, that despite all this kind of fundamental complexity, there's some kind of relative simplicity at the kind of less fundamental or macroscopic level. Okay, I can't wait to dive deeper into that topic, but first we have to take a quick break. Imagine that you're on an airplane, and all of a sudden,

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And in fact, it may get even worse. For more judgment-free money advice, listen to Brown Ambition on the IHeart Radio. app, Apple Podcast, or wherever you get your podcast. Okay, we're back and we're talking with philosopher Katie Robertson about whether the universe makes sense, whether there is a single theory of physics out there. And it certainly seems convenient that there are sometimes simple stories that you can tell. And I certainly get the argument that, like, sometimes those simple stories really are the answer. You don't necessarily want the totally microscopic picture in

every sense. But the thing that still puzzles me is why that's possible. And the thing I can't get over is the fact that sometimes it's not. You know, sometimes we look at systems and they are complex and our approximations fail and we can't find a simple story to describe the path of hurricanes or the fluctuations of the stock market. So it can't just be that we're looking at the universe and we're asking these questions. We're always able to find some simple story because we are not. So it makes me wonder why they emerge in some cases and not in others and specifically why they seem to emerge at various scales, right? Like you say, we organize ourselves into physics and biology and chemistry conferences. Is that because of some human interest in the way the universe works at these

levels? Or is because the universe itself, you know, reveals itself and simple stories only at some scales and not at some other scales. If aliens are doing science on some other planet, are they also doing physics and biology and chemistry and having separate conferences the way they don't talk to each other, or do they have like a different ladder of sciences completely because of their own history? Is there any way to grapple with those questions to try to get the sense for why science seems to be simple at some scales and not at others? I mean, it's a really interesting question, right? The philosopher Derry Fodor, I think, said something. I'm going to fudge the quote a bit, but it's something like I expect to find out the answer to why we have something

other than physics. Why isn't it just that we only have to do physics and we don't have all these kind of different scales and levels of which we talk about the world. He said something like, I expect to find out the answer to that question the day after I find out why there's something rather than nothing. You know, he kind of said, this is a bit of an a million dollar question. To my mind, I think there's going to be lots of different answers to that, rather than one answer that fits the relations between all different scales. So I think, as is often the case, in understanding how different scientific theories are

related to each other, the devil's going to be in the details. So I think in the case of thinking about how we get kind of directed processes, like the cup of coffee cooling down or a glass smashing on the floor, you know, these processes that we think of as being directed in time, how we get that kind of macroscopic pattern out of the microscopic pattern in, so how we get that from the microdynamics and how we get to statistical mechanics. That's one area where I think we can see how that happens, how we get that emergence. So what we have in that case is that we have a description that's really kind of detailed at the lower level, right?

there's 10 to the 23 molecules and a gas, that's a very complicated description. We can, instead of talking about exactly where molecule 553 is, we can talk about some kind of average properties of the gas. So we can talk about kind of what's sometimes called a kind of coarse grain probability distribution. Instead of following exactly where every single particle is, we just say, on average, there's kind of an even smearing of them across the box, for instance. And this kind of coarse graining type procedure is a bit like averaging, you know?

you throw away some of the details and sometimes when we do this we can kind of uncover new patterns we can see that oh actually there's kind of a rule about how this kind of new variable that we've defined works and sometimes when we do that when we kind of abstract sometimes you might think of it as being you know you're kind of throwing away some details it's a bit like abstracting to a new variable sometimes then we find a new law in terms of that variable so in the case of statistical mechanics, we can throw away information about the correlations, the kind of three or more particle correlations, so the gas molecules are all bumping into one another and getting correlated with each other. You can throw away the kind of three or more particle correlations in such a way that you can

find a kind of new dynamics at the higher level where you've kind of got this coarse-grained dynamics. So something like the Boltzman equation, which tells you how quickly gases relax the equilibrium. That is kind of an example of these coarse-grained irreversible dynamics. So in that case, you've got kind of like a detailed story about how you've got this higher level emergent kind of description and structure in those specific cases. But, you know, there's also other cases where you might use something a bit like coarse graining to kind of throw away the details that you're not going to be able to follow. And sometimes that's good and you can find an equation. Sometimes you can't. Like there's loads of really bad ways to average or coarse grain

and that you get nothing out of them right. Like it's, there's this nice quote by this, I think he's a historian of physics Van Kampen who says it's kind of the arse of the physicist. to find the right variables rather than a kind of science of like exactly how should we choose to coarse grain? There's the kind of a lot of bad choices you could make. But in some cases you can find a kind of closed form dynamics. And that's the case where we think we found like a new pattern.

But you make it sound like a discovery. Like you're stumbling over something. You're like, aha, look, the universe is doing this thing, right? Like we're coming into it. We don't necessarily understand why it's possible to go from statistical mechanics, description of all those tiny little particles to like, you know, thermodynamics of gases and all this kind of stuff. And you use the word abstraction, which I find

really interesting because it tells me that we're like summing up a lot of details. We're saying forget all the details of what's in here. I'm just going to call this thing a ball and treat it like a point. Right. We're like abstracting away a lot of the details. And how do we know that that kind of abstraction isn't sort of arbitrary or cultural, right? Is that us imposing our view on the world? Like, oh, this is interesting? That's not interesting. I want to tell a story about this. Or is it the universe coalescing around something, like, again, I wonder whether that's us forcing our sort of mental structure on the universe because we can't possibly process all the details or if it's really that we're discovering this in the universe. How could we possibly know

the difference between those two scenarios without, of course, talking to aliens about their science? If only we can meet those aliens, eh? So people used to think that it really was due to like our not being able to kind of keep a hold of all of the details that we use the kind of cross-graining procedure in statistical mechanics. So people thought that the way we chose the kind of averaging technique was according to like what we could measure. You know, we can't precisely know exactly where each molecule is in the in the box of gas. Then they use that as the kind of motivation for why we could then cause grain and throw away some of the details because we couldn't measure it. But I actually think that's a really bad explanation of why we cause grain because it's not like

as we've got better at measuring things. We use different cross-graining schemes in statistical mechanics, right? So it's not really linked to what we can see in inverted commas, because if it were, then as what we see can change, we would change kind of averaging or course-graining technique. So I think that for that reason, it's more like the kind of discovering new patterns way of thinking about it rather than it being kind of connected to us, because if it was connected to us and we change, we'd expect to see a change connected to that. So yeah, in the in the Stap-Met case, they really did think that, you know, there's some amazing quotes about the time asymmetry that kind of comes out of it. I think somebody described it. I think progeny and steng as described it is

elusory, the kind of resulted kind of entropic asymmetry that you get. Somebody else says it's kind of anthropocentric, you know, it's a feature of us. But I think that once we understand that these techniques that we have of abstracting and throwing away details, they're not necessarily connected to what we know about the world in any kind of detailed way. It might be that we're kind of attending to certain features of the world rather than other bits. You know, we're not focusing on all the the kind of, ah, what's this particle doing over here and what's this one doing over here? And I'm going to keep a track of what every single particle in the gas is doing. I'm just going to be interested in a bit more of a zoomed out way.

But that zooming out isn't, I think, connected to kind of our perspective on the world. This is interesting sort of second class nature to things that emerge that we talk about all the time in particle physics. I never really thought of it as like, you know, derogatory. But, you know, there's recent ideas about how space itself and maybe even time are not fundamental to the universe, they emerge, meaning that you could have a universe without space before the quantum bits have woven themselves together into reality, or you could have a universe without time. And it sort of like demotes those things and says they're not essential, they're not

fundamental. And it seems to me like you're making the argument that it shouldn't be a demotion, that there's just like, you know, there's a set of these ideas and different ones are applicable in different places and different contexts, but we shouldn't think of the most fundamental is necessarily the most primary or the most true. Is that fair? Yeah, I think so there's a kind of tendency sometimes in philosophy to really only focus on the fundamental. I mean, I guess Anderson in his famous Morris different paper, right, was kind of pointing to a similar tendency within physics. He was saying, look, it's really important to look at these other areas of physics, not just fundamental physics. And yeah, I think it's right to think of these non-fundamental things

as not kind of second class in that way. Not just because, you know, cadets matter physicists want funding too, but because they're also telling us true things about the world. And it's kind of an interesting conundrum, I think, connected to the one that you mentioned at the outset about, you know, why isn't it all not just kind of buzzing confusion? Why do we get the simplicity? I think an interesting question is, you know,

even if we were to understand the very kind of fundamental nature of the world, there'd still be so much we didn't know, right? Like we wouldn't understand, I don't know, stereotype threat in psychology or something, you know, like it's not like just knowing about the fundamental is enough to give you the kind of knowledge of all these other levels. So I think, yeah, understanding these levels is really important as well. Yeah, I think you're right. And obviously, even if we had like string theory or the most fundamental theory, it wouldn't tell us, you know, how do you raise your children or how do you make chicken soup, you know, or even where is the ball going to fly when somebody hits it with a bat? And I think maybe the primacy comes because some people, not everybody, are interested in the most fundamental questions. They want to know what is the most fundamental picture of the universe, even if that is not relevant to our everyday lives and to important questions like, how can we build a faster computer, you know, et cetera, et cetera. Something that confuses me. about these non-fundamental theories, these effective theories, you know, the ones that work so well, fluid dynamics and galaxy formation, is that they feel sometimes inconsistent. You know, like, for example, the basic equation of fluid dynamics, the Navi-Stokes equation, makes this assumption that the fluids you're describing is continuous, that it's explicitly not made of tiny little

bits, like of sand. But of course, we know that they are, right? So shouldn't theories, like, fit together more smoothly? I mean, I love how Newtonian theory is an extreme case of Einsteinian theory, but that seems like, is that maybe an exception? Because in other cases, you know, the assumptions you have to make at different levels are incompatible. It gives me a sense that science is more like, you know, a disjoint patchwork than really like a smooth idea that you're like shining a lamppost on at different scales. Yeah, no, that's a really interesting question because, you know, one way to get around the fact that the Navia Sto's equation says that everything is continuous is to say, well, instead of it's saying this false thing, we're just going to say that it shouldn't say

anything about that, you know, we should just reinterpret it as like not committing either way. And so you can kind of think of a kind of selective approach to your theories, you know, like some bits of the, we shouldn't take too seriously. And that's often what people have thought about old theories, right, which is they got some bits right and they got some bits wrong. And one way that sometimes people like to think about the words, which comes back to this question of how our theories relate to the world is that not everything they say is correct. So some people want to say that the really important thing that's kind of continuous

between the different scales and continuous across and theory changes, the kind of mathematical structure. The kind of extra details about what the kind of furniture of the world is like, whether fluids are continuous or not. That's the kind of thing where historically they've got it a bit wrong. But normally the mathematical equations are at least approximately the right thing. So this is sometimes called like structural realism. And the idea is that instead of kind of taking your scientific theory at its word, you should really only be committed to the kind of mathematical structure of the theory. But doesn't the math come out of these assumptions?

Like you start from these assumptions and then you can build the math on top of them. And then like the axiomatic foundations of the theory, right? How can you have the math without the foundations? Well, this is a kind of a tricky question for the structural realist, right? They want to say the laws we're getting those right. But what the kind of objects in that those laws are, we're not quite so sure about. I mean, quantum mechanics is a kind of clear case for this, right? Like, we're really confident about the surrounding equation exactly what quantum particles are like.

You know, you're going to end up with a big disagreement when you have a group of this is disgusting. So the idea is that it's kind of like epistemic security, you know? You don't want to put your neck over the parapet too much. You've got to just commit to the bits of your theory that you think are really kind of secure and good. And maybe these are these assumptions about, for instance, fluids being continuous are kind of the kind of ladder or scaffolding that helps you get to your theory, but you can kind of kick away afterwards and say the thing I'm really confident about and I think is getting at the nature of the world is the kind of equations and the math, but everything else I'm going to just not commit too much to. Well, I can't be too critical of that kind of strategy since as a particle physicist, I couldn't even really tell you what is a particle after all. right. And you're right, we certainly do a lot of particle physics and we collide them and we describe them and we have excellent descriptions of them without even really knowing what it is

we're talking about. So I definitely have very little ground to stand on there. Okay, I have a lot more questions for you, Katie, but first we have to pause for another quick break. Imagine that you're on an airplane and all of a sudden you hear this. Attention passengers. The pilot is having an emergency. And we need someone, anyone, to land this plane. Think you could do it? It turns out that nearly 50% of men think that they could land the plane with the help of air traffic control. And they're saying like, okay, pull this.

Do this, pull that, turn this. It's just, I can do my eyes close. I'm Mani. I'm Noah. This is Devon. And on our new show, no such thing. We get to the bottom of questions like these. Join us as we talk to the leading expert on overconfidence.

Those who lack expertise lack the expertise they need to recognize that they lack expertise. And then, as we try the whole thing out for real, wait, what? Oh, that's the run right. I'm looking at this thing. Listen to no such thing on the Iheart radio app,

Apple Podcasts, or wherever you get your podcasts. I had this overwhelming sensation that I had to call it right then. And I just hit call. I said, you know, hey, I'm Jacob Schick. I'm the CEO of One Tribe. And I just wanted to call on and let her know there's a lot of people battling some of the very same things you're battling. And there is help out there.

The Good Stuff podcast, season two, takes a deep look into One Tribe Foundation, a non-profit fighting suicide in the veteran community. September is National Suicide Prevention Month, so join host Jacob and Ashley Schick as they bring you to the front lines of One Tribe's mission. I was married to a combat army veteran, and he actually took his own mark to suicide. One Tribe saved my life twice. There's a lot of love that flows through this place and it's sincere. Now it's a personal mission. Don't have to go to any more funerals, you know. I got blown up on a React mission.

I ended up having amputation below the knee of my right leg and a traumatic brain injury because I landed on my head. Welcome to Season 2 of the Good Stuff. Listen to the Good Stuff podcast on the Iheart Radio app, Apple Podcast, or wherever you get your podcast. Your entire identity has been fabricated. Your beloved brother goes missing without a trace. You discover the depths of your mother's illness, the way it has echoed and reverberated throughout your life, impacting your very legacy. Hi, I'm Danny Shapiro, and these are just a few of the profound and powerful stories I'll be mining on our 12th season of Family Secrets.

With over 37 million downloads, we continue to be moved and inspired by our guests and their courageously told stories. I can't wait to share 10 powerful new episodes with you, stories of tangled up identities, concealed truths, and the way in which family secrets almost always need to be told. I hope you'll join me and my extraordinary guests for this new season of Family Secrets. Listen to Family Secrets Season 12 on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts.

Hey, Sess, what if I could promise you you never had to listen to a condescending finance, bro, tell you how to manage your money again. Welcome to Brown ambition. This is the hard part when you pay down those credit cards. If you haven't gotten to the bottom of why you were racking up credit or turning to credit cards, you may just recreate the same problem a year from now. When you do feel like you are bleeding from these high interest rates, I would start shopping for a debt consolidation loan, starting with your local credit union, shopping around online, looking for some online lenders because they tend to have fewer fees and be more affordable. Listen, I am not here to judge. It is so expensive in these

streets. I 100% can see how in just a few months you can have this much credit card debt and it weighs on you. It's really easy to just like stick your head in the sand. It's nice and dark in the sand. Even if it's scary, it's not going to go away just because you're avoiding it. And in fact, it may get even worse. For more judgment-free money advice, listen to Brown Ambition on the IHeart Radio app, Apple Podcast, or wherever you get your podcast. All right, we're back and we're talking to Dr. Katie Robertson, a philosopher of physics about whether it's possible to understand everything in the universe with a single theory. I want to take us in another direction, which is sort of further down the skeptical road.

You know, if each science is helping us understand a part of the world, and if we say, you know, each one has their own area of validity, is it possible that we can eventually, stitch them together to get a holistic understanding of the underlying truth, the idea of being like, the more lampposts you turn on, the more ground truth you're revealing. And I'm reading this book by Nancy Cartwright, who has this school of thought. Her book is called the dappled world. And she seems to be arguing that there might not be unity to science, that there isn't a whole truth underneath it that we're revealing, that each piece could actually be separate and not link up into a coherent picture. Frankly, as somebody who's born and bred as a particle

What is this? I struggle to comprehend this argument. What is the argument here? Can you walk us through how to get to that sort of state of mind? So I guess you can think of there as being kind of two issues. The first is we seem to go about doing science in this very kind of institutional each, in this kind of patchwork way, right? I think she describes it as like, you know, some of the edges line up neatly. Others are kind of frayed and they don't quite connect. You know, some disciplines really do kind of fit together with each other in a nice way and other ones that's a bit more complicated. But we seem to get away with doing things like that. So there's kind of one question which is like how can we do that if really everything is made up of whatever the most fundamental stuff is.

Why did we get away with ignoring those details? Which is kind of the question that we started with. And then the question on the kind of other side is if you think that all these patches don't line up, they're not unified, then how come we sometimes have processes like kind of you can think of like a causal process as leaping across patches and when we have, for instance, MRI scans for the detection of disease, for instance.

That seems like a case where we can't say, or biology is just about something like totally different from physics because if that were the case, why would physics be so useful in understanding things in biology? So I kind of see the patchwork view as giving you an easy answer to the first problem. Why is there all these different things at different levels? Well, there's just different things going on at different levels. There isn't this kind of unified picture. So it kind of gives you an easy answer to that question. But then you have a hard answer,

which is, well, if these patches are kind of insulated from each other in this different way, why is it that there's these kind of, it looks like kind of causal processes going between them or kind of threads running through different sciences? So I think that's part of the motivation but another key part of Cartwright's picture is that she has a I think it was the book before the daffled world a book with the title how the laws of physics lie clickbait clickbait it's from the 80s but definitely you know the original philosophy of physics clickbait where she argues that you know our laws are so abstract and they apply in such tightly controlled situations in the lab that we're used to kind of screening off the kind of noise

from the environment, but really kind of out in the wild, the law is kind of a lawless land. You know, the laws that we're used to having, we have no kind of good reason for thinking that they would carry across, which is a view I find hard to stomach. And like you, I like the idea of there being kind of the different things happening at different scales, but I still like thinking that it's kind of all connected in, there's these kind of links between them. Things emerge out of other things. And I'm the fan of the more emergentist type. view than the patchwork view, but that's that's the motivation, I think. Well, I'm sort of shocked that you describe the picture of there are just different rules for different situations as sort of like

the easy answer because that like rocks me to the core. I have a hard time understanding like within what is the universe, right? Like how does it decide when to use one set of laws and another set of laws? In Cartwright's book, she has this quote which when I've read this, I'd like drop the book. I couldn't believe it. She says laws of nature are limited in their range in regions that seem to overlap. there may be no rules at all for composing the separate effects. And some situations may not be subject to law at all. What happens happens by hap, which is like, is the universe whimsical? Is it just like making stuff up as it goes along?

I mean, that's certainly not my experience of the universe and the experience of experimental physics for hundreds of years. Is this sort of like an exercise in skepticism by Cartwright? Like, how do we really know that the laws that we, Isolate in the laboratory also apply out on the wind-blown streets? Or do you think this is really a coherent philosophical position and one that's revealing our blinders are, you know, the assumptions we've been making about the universe because of the way our minds work? It's an interesting question because I think that I kind of have the same gut feeling as you

in that when you think about describing a bigger and bigger system, there's no point where you're told, no, don't take the tensor product of those two systems together. The Schröding equation, it will stop working. so it doesn't seem to really go with what our experience of doing physics is like but cartwright is very sensitive I think to the kind of details of the practice of physics in a way that sometimes philosophers have just assumed well there'll be a theory that applies to everything and they've sort of just taken it as this kind of brute fact about the way the world is and I guess I see her as sort of like poking at that assumption and saying well how well

justified is that. I think that, yeah, my, my hunch is that I think that we, we can say that it's a well justified assumption in that it hasn't not worked so far. But equally, I guess, I think her emphasis on just how tightly controlled certain experimental contexts are and being careful about kind of exporting that to other cases, I think is an important thing. And I think at the beginning of her book, she has this kind of way of casting what she's doing as a sloth different approach. So she says there's kind of two enterprises that science is involved in, representing the way the world is, and then intervening on the world. And obviously other sciences, like medical sciences, are obviously really interested in the

intervening part. In medicine, sometimes we know exactly what to do to help something. We don't necessarily know all the mechanisms behind it that mean that that works. But we know how to intervene on the world, sometimes at least. And she kind of puts her in the camp of saying, I'm not as interested in the project of representing the world. I'm more interested in the project of intervening on the world. But she worries that the, by not thinking about how we want to intervene on the world, the fact that that's what she's interested in, she has these kind of lovely pictures where she shows like the, maybe you remember them from the beginning of the book, where it's like kind

of like the messy house and then the tidy house. And she kind of says, we need to understand that the world is kind of messier than we think that it sometimes is. Because then when we want to intervene on the world, we're going to be more successful in doing so because we've got a kind of better idea of what things are like. So I think that's part of her motivation. But I think at the end of the day, I still come down on there. I think that it's much more connected.

I don't think there's these kind of lawless lands between the factors of laws. So yeah, I think I'm with you on that one. Well, one of her examples that I found really interesting was thinking about how to apply physics in the real world. right? And so you take a coin, for example, and it drops and you can say, well, a coin is mostly described by F equals M.A. It's mostly just dominated by gravity. It's fairly simple situation. And I guess this is the kind of thing she would say is essentially screened off from the other details. But if instead of dropping a coin, you drop like a banknote and it's a windy day, then could you possibly ever describe the motion of that banknote using F equals M.A.

It's like this bit of wind and that bit of wind and the other bit of wind. And in that situation, like, you know, one might ask, is it just too complicated? and it's a lot of different sums, or is it really not described by any physics at all? And I guess her point is you can't ever really tell, right? You know, in the absence of a model that yields accurate predictions, we have no grounds for thinking that any particular law applies, is another quote from her book. And I guess I find that useful as like a warning. Like, keep in mind you don't really know how to solve most of the situations in the world

outside of your well-controlled experiments. But, you know, we also have this history in physics of success. You know, we build transistors in the laboratory and then they fly airplanes that mostly don't crash, right? Out in the complicated world. And in the history of physics, we see this like unification where electricity and magnetism come together. We add the weak force. Maybe in the future we'll be able to combine that with the strong force and gravity. It seems to me like argument of history, at least, is against her.

Is that the view mostly in mainstream philosophy or is there a camp of people who are continuing this work? So there are a camp of people that are continuing in the kind of cartwright line of thinking. I think you're completely right to kind of characterize it as a kind of epistemic humility warning, you know, like you don't have a warrant to say that it's definitely going to work. So I think that's a really important part of the project. And but then on the other hand, you're kind of a torn in the other direction, which is, well, we've not yet found a situation where that doesn't, you know, for objects of the size of a banknote, just summing up all the forces. on it. It doesn't work as a way of predicting what happens. Okay, maybe we won't ever be able to do it in the case of the banknote because it's just too complicated. But I guess that's the kind of warrant that we have kind of holding onto the idea that maybe we can't predict it. But it is

like predictable in principle. Maybe aliens with their supercomputers have totally solved that problem. All right. So then my last question for you is aside from meeting aliens with super advanced answers to questions in physics and philosophy, what do you think are prospects for making progress on these questions. I mean, we can't ever really understand how a banknote flutters in the wind. Are we going to be able to figure out if there is a grand unified theory out there for us to work towards or if the universe is really just a patchwork? How do we understand these things? Our philosopher is going to be arguing about this for a thousand years. Are we actually going to figure this out? I guess we don't, I don't know really because I think that it comes back to like whether

we think that even if it's going to be a patchwork, even if, you know, sometimes people will say it's kind of turtles all the way down, you know, we could just keep smashing particles together and finding new particles forever and ever, you know. You make that sound like a bad thing. That sounds like job security for me. In that case, then, we would expect kind of, you know, that there isn't a fundamental level. That would be a bit like saying, perhaps. I'm tempted to think that even if we, that could be the case or it could be a patchwork,

we still, the best methodology that we would have is to keep looking for kind of more fundamental theories and I think working out how everything kind of patches together, you know, is it going to be that we always have a kind of effective theory that works within a certain domain and then a more fundamental theory underlying that? It'd be really interesting. I mean, there's particularly for the case of things like black holes, there's fascinating questions of how things will turn out. So I'm tempted to think whilst at the moment I'd place my bets on it not being a patchwork in the sense of there's lawless kind of lands, between the patches, it'll be interesting to find out.

Well, I like the way you describe the arc of science there. We're like discovering, we're letting the universe tell us its story. And I just hope that we're not too biased by the way we're listening to the story, to the story we want to hear that we are able to absorb, you know, the shocking truth of the universe. Because it sometimes takes us, you know, decades or centuries to really come to grips with what the experiments are telling us. It's hard to deviate sometimes from sort of like the historical path of science.

And, you know, I'm not sure I can pronounce that phrase epistemical humility. We should try to maintain that as much as possible, but also make progress on the science at the same time. Well, thanks very much for joining us today on the podcast and for talking about these really important, but also very abstract questions about the way we do science. Oh, thanks for having me. Thanks, everyone for listening. Tune in next time. Thanks for listening.

Hey, Explain the Universe is a production of IHeartRadio. For more podcasts from IHeartRadio, visit the IHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. In session 418 of the Therapy for Black Girls podcast, Dr. Angela Neal-Barnett and I discuss flight anxiety. What is not a norm is to allow it to prevent you from doing the things that you want to do, the things that you were meant to do. Listen to Therapy for Black Girls on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast. From Tips for Healthy Living to the latest medical breakthroughs, WebMD's Health Discovered podcast keeps you up to date on today's most important. health issues. Through in-depth conversations with experts from across the health care community, WebMD reveals how today's health news will impact your life tomorrow.

It's not that people don't know that exercise is healthy. It's just that people don't know why it's healthy, and we're struggling to try to help people help themselves and each other. Listen to WebMD Health Discovered on the IHeart Radio app or wherever you get your podcasts. Your entire identity has been fabricated. Your beloved brother goes missing without a trace. You discover the depths of your mother's illness. I'm Danny Shapiro, and these are just a few of the powerful stories I'll be mining on our upcoming 12th season of Family Secrets. We continue to be moved and inspired by our guests and their courageously told stories.

Listen to Family Secrets Season 12 on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. This is an IHeart podcast.