Daniel and Kelly’s Extraordinary Universe - Could there be more than one theory of everything?

Episode Date: June 15, 2023

Daniel talks to philosopher Thomas Barrett about whether there could be multiple effective descriptions of our Universe See omnystudio.com/listener for privacy information.

This is an I-Heart podcast. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then, everything changed. There's been a bombing at the TWA terminal. Just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, terrorism. Listen to the new season of Law and Order Criminal Justice System

On the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Wait a minute, Sam. Maybe her boyfriend's just looking for extra credit. Well, Dakota, luckily, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend's been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now he's insisting we get to know each other, but I just want or gone.

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One Tribe, save my life twice. Welcome to Season 2 of the Good Stuff. Listen to the Good Stuff podcast on the Iheart Radio app, Apple Podcasts, or wherever you get your podcast. As I sit down to record today's episode, the Bees, Games of the Large Hadron Collider are powering up. Today will be the first day of collisions in 2023 after a long shutdown to rebuild and upgrade our equipment. We hope to see some new particles, to discover some new forces, to reveal new truths about the nature of space and time and matter and energy. But what if everything we're learning is not really the truth?

What if it's just a story we tell ourselves? What if there are other possible stories that could also explain what we see in our experiments? How would we ever even know? Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine and something of a amateur philosopher of science. Welcome to the podcast, Daniel and Jorge explain the universe, a production of iHeartRadio, in which normally we dig into questions about how the universe works, what's going on, the tiny quantum level, what's at the heart of black holes, how does everything work and how does it weave itself together to make the incredible, beautiful, mysterious

reality that we experience. My normal co-host and friend Jorge can't be with us today, So I'm going to take the opportunity to do something a tiny bit different and do a bit of a dive into some questions in the philosophy of science because I really want to understand not just what's happening in the universe, but why it's happening. What are we really learning about the universe when we do particle physics or when we do any kind of science? Let's make it concrete. For example, when we build a large haydron collider, we did it because we wanted to uncover how the universe works. We want to know answers to questions like, is space filled with quantum fields that buzz and wiggle to make particles? How many those fields are there and what are the rules of their interactions? Is there a field for dark matter?

Are there other fields that we never even imagined? Are all of the fields really part of one super field in a way we haven't ever thought of? These are the kind of questions we think about and the language of physics these days is always the language of these fields. We imagine that space is filled with these quantum fields, and we've been very successful at using these quantum fields to describe everything we've seen so far in particle physics. But there's always a but, but nobody has ever actually seen a field. What we see in the collider are sprays of particles that are the debris of collisions. The collision itself happens too quickly and is too small to actually look at. And even the particles that we do see don't prove that fields,

themselves actually exist. When a particle bends in a magnetic field, what we see is the particle bending, not the field itself. There's always this part of the story where the field hides behind a curtain. It's an unobservable part of our science that helps explain what we do see, but is never itself directly observable. And that unobservable part, in my opinion, is really important. It's the juicy bit. It's the real explanation for what's how. happening out there in the universe. It's how we think about physics. We hope or we wonder at least whether the universe is also doing its calculations to decide whether a particle goes this way or that way using fields. I mean, are those fields really out there pushing and pulling on stuff when we're

not looking? Or are those fields just our description? Are they part of the story we are telling ourselves about the particles. Are those fields just part of our minds? Are they really deep elements of the universe itself? One really fun way to probe this famous question is to wonder, could there be another explanation that's just as good? Is there another way to describe the universe that doesn't need fields that tells a different story about what's happening out there in the universe? So today on the podcast, we'll be asking the question. Could there be more than one theory of everything? To help me dive into these thorny questions of philosophy,

I've invited a guest expert, a philosopher of science who specializes in this very topic. Okay, so then it's my great pleasure to welcome to the podcast, Professor Thomas William Barrett. He's Professor at University of California, Santa Barbara, in the Department of Philosophy, and he works mostly on philosophy of science and logic. And on his homepage, he has a picture of himself as an infant in front of a philosophy building.

Welcome to the podcast. Thanks very much. Thanks for having me. It's a pleasure to be here. So does that picture mean that you've been doing philosophy since you were a little baby? In a sense, I guess. I mean, everyone tries not to, but it's, you know, unavoidable. We've all been doing philosophy since we've been little babies. Well, I know people like to say that babies are scientists because they're all exploring the world and trying to figure out how things work, but it would be fascinating if babies were also philosophers of science at the same time. Yeah. I mean, honestly, in that picture, if you look closely, I have my little truck in my hand. At the time, I was probably more interested in the truck than in philosophy. Some days now, it's not to feel the same.

All right. Well, before we dig into the topic of the day, we'd like to get to know you a little bit about where you stand in the sort of big question. of philosophy. So let me ask you what I think is maybe the most important question in philosophy of science, which is, in your opinion, does a Star Trek transporter actually move your molecules from one place to another, or does it kill you and reassemble you somewhere else? Is it a murder machine or a teleportation device? I don't know, man. Your guess is as good as mine. I'm more of a Star Wars guy than a Star Trek guy. I honestly, I think I've only seen like maybe one or two, Star Wars. track episodes i saw one of the movies when i was a kid but i'm not a star trek guy it's been star wars all the way all right well then what's the deepest question in philosophy raised by

star wars is it whether the force actually has a scientific explanation or if it's just magic yeah um science versus religion questions right yeah that's good stuff way above my pay grade something a little bit easier here on the podcast today. What I want to talk about drills into the question of like what are we learning when we are building our theories of the universe. You know I'm a particle physicist and here in our department we try to or we imagine that we're trying to build a description of what's happening in the universe that when we draw a little Feynman diagrams we're describing what we think the universe is actually doing when electrons fly through

the universe. But you know most people who think about science who aren't philosophers, naturally imagine that that's what's happening. You know, they think that what we see is real, that the electron is real, not just something that we use to do calculations. If you tell most people out there, like, look, we don't know if an electron is actually real or if the theories that we use to describe them are just things in our head. In your view, what's the sort of easy way to understand or to approach the question of like, what are we learning? Does it represent what's actually out there or just things that we are imagining, you know, without going full matrix sort of theory? on you. What's an easy way to access this question in your view? The question is something like the extent to which our theories should be taken as literally

true in their description of the world and whether our evidence for, you know, taking our theories is literally true, whether the success of our theories gives us good reason to think, then they are true in all their aspects, not just like the stuff that they say about what we can see, but the stuff that they say about what we can't see. So some folks in philosophy think that the empirical success of our theories, and they are like incredibly empirically successful, our best theories, gives us good reason to just take them literally as true in all their aspects. Other folks in philosophy think that the proper conclusion is something more modest. Like, you know, it isn't an anti-science stance or anything, but it's just the empirical

success of our theories doesn't tell us that we should take them as literally true, both in describing stuff that's observable and in describing stuff that's not observable. It just tells you that theories are successful and they were designed in such a way to be successful. But are they uncovering deep truths about the, like, unobservable structure of the universe? Maybe not. Yeah, that's a great way to think about it, the observable versus the unobservable. And I imagine people might be thinking like, what do you mean unobservable? We do experiments, we see stuff, we know electrons are out there. What's a good way to think about the unobservable sort of side of science? One thing I sometimes imagine is like the fields themselves. We talk about on this podcast

a lot how space is filled with fields and sometimes those fields ripple to make particles or they ripple to make photons, et cetera. But those fields aren't something we ever direct. interact with, right? Is that an example of something that's unobservable but a part of our theory? Yeah, that's right. So, like, another good example comes from the history of classical space-time theories. So you can think about Newton's old theory of gravitation. So according to Newton's old theory of gravitation, space-time is flat. So, like, the arena in which events take place is flat. It doesn't have interesting geometrical features. And according to this theory, gravity is best described as a force. So it's a field on your flat space time. And Newton's

old law of universal gravitation then dictates how particles, massive bodies will move around depending on what this force is at different points in space time. It turns out that you can do this theory. This was discovered by folks in the early 20th century. You can do Newton's theory, but on a space time that's curved. So in this alternative formulation of Newtonian gravitation, space time is curved. Gravity manifests itself in terms of the curvature of space time rather than force.

And so now this raises an interesting question. You can't observe. You're not observing the structure of space time, right? What you're observing is how stuff is moving around in space time. And so now we have this question. Is space time curved or is it flat according to Newton's theory? There are ways to do the theory according to which it's curved, ways to do theory according to which it's flat, and they result in the same empirical prediction.

But how does the curved space Newton's theory different from Einsteinian general relativity? It is the same basic concept as Einsteinian general relativity, but it yields the same predictions as Newton's theory yields. Oh, I see. So Einstein's theory and Newton's theory, agree about, like, for example, the famous cases, they disagree about how Mercury would orbit the sun, right? This was the evidence that led us to Einstein's theory. Newton's theory was getting Mercury's orbit wrong. So the Newtonian gravitation theory set on a curve

space time, which folks will call Newton-Cartong theory sometimes, yields the same empirical predictions as Newton's theory, not as Einstein's theory, but it agreed with Einstein's theory on the status of the curvature of space time. I see. Okay, so you're saying that you could have two theories, Newton's original theory of gravity is a force and this other weird variation where gravity comes from the curvature of space time,

but it gives the same predictions as Newton's original theory rather than the predictions of Einstein's theory. And so then you're saying there's an unobserved part of the universe, the actual predictions of how things are going to move, and then there's the sort of behind-the-scenes mechanism. Is it a force or is it the curvature,

of space time, what's sort of happening behind the scenes to make that happen. So you're saying there we have like two theories with different descriptions for what's unobserved. That's right. Two theories, different descriptions for what's unobserved, but the same exact description of the observable stuff. That's really interesting. And that touches on the topic I wanted to dig into today, which is this underdeterminism,

this concept and philosophy that we might never actually reveal what's happening in the unobserved section of the universe, that there might be multiple ways to describe what's sort of happening behind the scenes that give the same exact empirical predictions that you could never distinguish in experiment, but be described by different ideas. And, you know, in the example you just described, we know that both of those are actually wrong, right? Newton's theory of forces and Newton's theory of current space or both give the same predictions, but they're wrong. But let's imagine some other scenario where we have a theory of quantum fields and some independent group of scientists have been working for a thousand years and they have a theory of quantum

schmields or something and they're fundamentally different they're not conceptually the same but they give the same predictions so i think it's fascinating to think about you know is it possible to have two really different descriptions of what's happening in the universe that predict the same thing that we would observe is that the sort of the fundamental question of underdeterminism and philosophy yeah that's exactly right like the underlying concept is simple and what will be familiar to all sorts of folks. Like, we find ourselves in situations just in our day-to-day lives where the body of evidence that we've gathered

doesn't help us adjudicate between two theories. So imagine, like, you come home and I've been to the grocery store. You're wondering what I bought at the grocery store. You see the receipt sitting on the dining room table, and you look at it. And all it says on the receipt is that I spent 18 bucks at the grocery store. you know that apples are like two dollars oranges are three dollars and now you're faced with a kind of underdetermination problem you don't know how many apples i bought how many oranges i bought the data you've got doesn't help me discriminate between the two theories the data doesn't tell you which one is correct whether i bought six apples or nine oranges or some combination and that's exactly the issue that the problem of underdetermination, that's exactly the issue that the problem of underdetermination alleges we often face in science where we have two theories and the data

doesn't help us decide which one is correct. The two theories are compatible with precisely the same data. And so what do we do in a case like that? What do we believe? Right. And in that scenario, it makes me feel uncomfortable that I'm being so nosy about your grocery store purchases. But, you know, in the case of the universe, I don't think the universe deserves any privacy. I think, you know, we are entitled to peek behind the curtain as much as we can and trying to figure out what's going on and what the universe bought at the grocery store. But what is the motivation for this? Is this just like, you know, philosophical meandering? Oh, what if? What could it be possible? Some sort of deep skepticism that maybe will never figure things. out? Or do we have, like, any sort of concrete examples of scenarios where we really had

two excellent theories that gave exactly the same predictions, but had different scenarios behind the scenes? Yeah, so let me give a couple of examples. The example we were discussing earlier of Newton's standard theory of gravity in which gravity is a force versus this geometricized version of Newton's theory of gravity, that's a famous historical case. Of course, as you say, we know neither of those theories is correct that make bad predictions. But the existence of an example like this should give one pause. It gives us reason to think that maybe the same thing can happen with our current best theories. So there are other examples from the history of physics.

Another famous one is the case of Hamiltonian and Lagrange. mechanics. Okay, suppose you formulate your theory of classical mechanics, Newton's old theory, describing how stuff will move around in space, by taking positions and velocities is fundamental. Okay? So you think that we should specify the energy conditions of a system by laying down something that you call a Lagrangian. So kind of a function on possible configurations of positions and velocities that dictates how active or lively these configurations are. Tell me how these properties of the system, the positions and velocities of particles in your system will evolve over time by laying down a set of equations that you call the Euler-LaGrange

equations. Okay. So call your theory Lagrangian mechanics. All right. On the other hand, for whatever reason, don't like velocity. I find momentum to be much more elegant, you know, it's conserved. And so I formulate my theory by specifying energy conditions of the system, laying down a Hamiltonian. I take positions in momentum as the fundamental properties that a system has, and I'll tell you how the system will evolve over time by laying down a different set of equations. So we'll call these Hamilton's equations, and they take in the Hamiltonian of the system. So the Hamiltonian describes something like the total energy of the system rather than its activity or liveliness, just total energy. I call my theory Hamiltonian mechanics.

So it turns out that our theories are empirically equivalent. So what do we mean by that? We mean there's no possible evidence that is compatible with my theory, but incompatible with yours, or vice. So first, like some set of balls or squirrels on roller coasters, they always give the same predictions for what's going to happen. Exactly. Your theory gives the same prediction as my theory. But it seems like we've done things differently, right? You care about positions and velocities. I care about positions and momentums. The laws of nature, one might say, according to your theory, are different than the ones according to my theory. Like we laid down different equations when we were saying, how the stuff was going to behave. And so one might think that we have a choice

to be made here between these two theories, but the evidence won't make the choice for us. So that's another example of a possible case of underdetermination, where two theories compatible with the same body of evidence, but it seems like we might have a choice to make between them. In cases like this, the standard conclusion to draw is kind of a skeptical thing. The things about which these two theories disagree, we now don't have answers to these questions unless we can decide which one of the two theories is correct.

And how do we make that decision? The evidence isn't going to make the decision for us. So we have to come to a decision by some other means. So we have two different sets of equations that give the same predictions for what happens to squirrels on roller coasters or billiard tables and these kinds of things. And you're saying that they really are different in some way. And because they always give the same predictions, there's no way to do the sort of famous scientific test of say, well, let's do an experiment and figure out the different hypotheses and the different predictions because they're always the same. But if they're always the same, how do we know that they really are different theories? I mean, I can take any arbitrary theory and say, I'm going to multiply everything by two, and then at the end, I'm going to divide by two.

And superficially, the equations might look a little bit different, but it gives exactly the same predictions, but not for an interesting reason, not because I'm really saying there's different stuff happening behind the scenes of the universe. So in the case of Hamiltonian versus Lagrangian mechanics, or really in the case of fields versus schmields or any comparison, what does it mean to say that they're different? How do we know that they're different, or is there some standard we apply to say, like, this theory is telling it different? different story about what's happening in the universe than that theory. How do we do that? Yeah, good question. So, like, the suggestion is something like, maybe in some of these worrying cases of underdetermination, you don't actually have two genuine rivals. They're the same theory just presented to you in different guises. And so, like, you don't have a real choice to make. It's the choice between one theory and itself just presented to you differently. So maybe let me tell a

a story, the most famous case of this, happened in the early years of quantum mechanics. Okay. So in brief, Heisenberg had his matrix mechanics. Schrodinger had his wave mechanics. And these guys, like, did not vibe. On the base of it, their theories seemed incompatible. So they made the same predictions, but they used, like, radically different mathematical apparatus to do so. So Schrodinger at one point quipped that Heisenberg's theory laughed

to visualize ability, to which Heisenberg shot back, I quote, what Schrodinger writes about visualizability is crap. We eventually, like, we realize that there's a way to translate between the Heisenberg picture and the Schrodenger picture and vice versa. And so there's a sense in which their two ways of doing quantum mechanics were not that different after all. The disputes between the two theories, like the stuff that they disagreed about, is just verbal. It was like they were having a verbal dispute. It was the same theory presenting in two different ways.

And so we don't have a troubling case of underdetermination there. A trouble in case of underdetermination was avoided because you didn't have a genuine decision to be made between these two. between these two theories. The two theories were actually one. So it turns out they were really just the same theory dressed in different clothes, right? Yeah, yeah.

And so this is something that one might be tempted to say in the case of Hamiltonian and Lagrangian mechanics, too. You know, you pick position and velocity to do your theory. I pick position and momentum to do my theory. But we can translate back and forth between these two descriptions. It's not like when you talk about position and velocity. I can't do that in my theory.

I would do it a little differently. I'd use a little bit of different language to do so. But am I saying a genuinely different thing about the world when I decide to say it in position and momentum language versus when you say it in position and philosophy language? So in general, this is a kind of response one can give to underdetermination worries.

It's like, look, in a lot of the problematic cases of underdetermination that we see. So cases where you have two theories that can't be discriminated between and on the basis of the evidence we've gathered, in a lot of these cases, you don't actually have genuine rivals. The two theories can't be discriminated

between on the basis of the evidence we've gathered, but they're the same theory. Yeah, and so what about the case of later sort of quantum mechanical interpretations? You know, we have various descriptions of what's going on with particles, is the wave function collapse, or is the universe splitting into multiple universes, or is the collapse relative, you know,

like in relational quantum mechanics, are those examples of true underdetermination where we have really different stories, genuinely different accounts of the physical world, but that make effectively the same predictions for what we could see in experiments? Like, we can't distinguish between the many-world hypothesis or the Copenhagen interpretation using experiments. Is that an example? That is an example where it becomes harder for, one to say that the theories are genuinely the same, and yet they have the same experience supporting both of them. So some folks at this point will say that there's another natural way to respond to problems of underdetermination. So the empirical evidence doesn't help us decide

which theory is correct. We have some stuff that we can use to make these decisions not using empirical evidence. And we do this all the time. We appeal to other virtues that a theory might exhibit like simplicity or exactly okay yeah um simplicity fruitfulness i don't know something like this uh and scientists do this all the time we all do this well i was having a conversation with a theoretical physicist who's quite well known in particle physics but i won't name him and i described this problem to him and he said well look if the theories predict the same things but are different then they're different only in the metaphysics right only in the irrelevant details, which I think is another way to say, like, well, maybe who cares, right?

Like we have two different descriptions of the universe, but they give the same predictions, what does it matter? And to me, this is sort of shocking coming from a theorist who, you know, I think their job essentially is to uncover the mechanisms of the universe, not just to produce calculational tools that will get us to the next step, but to like, reveal the nature of reality, man. And so to hear somebody be like, well, maybe it doesn't matter if it's fields or schmields. As long as the numbers are right, how does that strike you as sort of philosopher of science? Do you think that that sort of instrumental approach is a solution to this problem, or are we just avoiding the question? It's not an attitude that's uncommon among philosophers also.

I do think it's kind of avoiding, sidestepping the question rather than taking it on head on. One thing that I should say, though, is notice that this kind of attitude. So we have two theories. The evidence doesn't distinguish between them. So they're the same in all important aspects. There's a sense in which that attitude lands oneself in the same place as taking the problem of underdetermination seriously ends oneself in. The idea is if you don't think that the theories are telling you anything about the non-empirical

stuff, the unobservable stuff, there's a sense in which you're not taking your theories all that seriously. Right. Which is very close to the place that the problem of underdetermination would lead us in both cases. So say you don't take the metaphysics of your theory seriously. And there are some unanswered questions. Like is space flat or is it curved? Suppose you take the problem of underdetermination seriously. Then you end up with unanswered questions for a slightly different reason. But in both cases, you're ending with questions that are. not being answered by your physics. Okay, I have a lot more questions about how this all works

and how we can make sense of it. But first, let's take a quick break. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then, at 633, 3 p.m., everything changed. There's been a bombing at the TWA terminal.

Apparently, the explosion actually impelled metal glass. The injured were being loaded into ambulances, just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, and it was here to stay. Terrorism. Law and order, criminal justice system is back. In season two, we're turning. our focus to a threat that hides in plain sight that's harder to predict and even harder to stop listen to the new season of law and order criminal justice system on the iheart radio app

apple podcasts or wherever you get your podcasts my boyfriends professor is way too friendly and now i'm seriously suspicious oh wait a minute sam maybe her boyfriend's just looking for extra credit well dakota it's back to school week on the okay story time podcast so we'll find soon. This person writes, my boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now he's insisting we get to know each other, but I just want her gone. Now hold up, isn't that against school policy? That sounds totally inappropriate. Well, according to this person, this is her boyfriend's former professor and they're the same age. It's even more likely that they're cheating. He insists

there's nothing between them. I mean, do you believe him? Well, he's certainly trying to get this person to believe him because he now wants them both to meet. So, do we find out if this person his boyfriend really cheated with his professor or not. To hear the explosive finale, listen to the OK Storytime podcast on the Iheart Radio app, Apple Podcasts, or wherever you get your podcast. I'm Dr. Joy Harden Bradford. And in session 421 of therapy for black girls, I sit down with Dr. Ophia and Billy Shaka to explore how our hair connects to our identity,

mental health, and the ways we heal. Because I think hair is a complex language system, right? In terms of it can tell how old you are, your marital status, where you're from, spiritual belief. But I think with social media, there's like a hyper fixation and observation of our hair, right? That this is sometimes the first thing someone sees when we make a post or a reel is how our hair is styled. You talk about the important role hairstylists play in our community, the pressure to always look put together, and how breaking up with perfection can actually free us. Plus, if you're someone who gets anxious about flying, don't miss session 418

with Dr. Angela Neil Barnett. where we dive into managing flight anxiety. Listen to therapy for black girls on the Iheart radio app, Apple Podcasts, or wherever you get your podcast. Get fired up, y'all. Season two of Good Game with Sarah Spain is underway. We just welcomed one of my favorite people and an incomparable soccer icon,

Megan Rapino to the show, and we had a blast. We talked about her recent 40th birthday celebrations, co-hosting a podcast with her fiancé Sue Bird, watching former teammates retire and more. Never a dull moment with Pino. Take a listen. What do you miss the most about being a pro athlete? The final.

The final. And the locker room. I really, really, like, you just, you can't replicate. You can't get back. Showing up to locker room every morning just to shit talk. We've got more incredible guests like the legendary Candice Parker and college superstar AZ Fudd. I mean, seriously, y'all. The guest list is absolutely stacked for season two.

And, you know, we're always going to keep you up to speed on all. the news and happenings around the women's sports world as well. So make sure you listen to Good Good Game with Sarah Spain on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Presented by Capital One, founding partner of IHeart Women's Sports. All right, we're back and we're talking to Professor Thomas Barrett about the question of underdeterminism in philosophy of science, whether it's possible to have to have. have multiple theories that describe the universe and make exactly the same predictions but are in themselves fundamentally different. Well, in almost every example we've considered various

versions of Newtonian theory, et cetera, one can sometimes imagine that maybe in the future or somebody will come up with an experiment to help us distinguish or somebody will be the next generation's John Bell will come up with a super clever quantum experiment to help us reveal multiple universes or something. Even in the face of like infinite data is the art argument for underdetermination, really that it's impossible that, you know, even in a million years, a billion years of super Einstein's could never come up with a way to distinguish that it might literally be impossible, not just we haven't figured it out yet. Yeah, so think back, this is a good question, think back to the apples and oranges

example. The natural thing to say in that example is like, look, dude, you just looked at the receipt on the table. You want to know how many apples and oranges that? there are, like, go open up the pantry, right? Like, gather more data. So problematic, like truly problematic cases of underdetermination, you know, the ones that will keep you up at night are cases where even gathering the more data won't help you distinguish between the two theories. Just the two theories are provably compatible with precisely the same collection of evidence.

So maybe one example. one further example will be helpful here. So this comes from our recent work that my good friend and your colleague at Irvine, J.B. Mancheck has done in general relativity. So what Mancheck proves is that in the context of Einstein's theory of gener relativity, we encounter a very pernicious kind of underdetermination. So here's the gist. In general, intuitively, like since signals can't travel faster than the speed of light, you can only know at most everything that goes on in your past light comb. So all the evidence that's available to you at some time will have arrived to you via some trajectory that's contained in

your past light come, right? Like picture the stuff that you see, well, that's arriving to you via photons that are reflecting off the surface of the thing. They're all contained in your past light cone. Your colleagues reporting to you their evidence. Well, they came to you in your past light cone. So Manchuk proves now that given this information, there's always more than one model of the universe, according to Einstein's theory,

that's compatible with the data you could have possibly gathered. Meaning that there's always part of the universe sort of shrouded in darkness because it could be just like so far away given the age of the universe that there hasn't been time for light from it to arrive here. And so there could be huge purple dragons beyond the edge of the universe or not huge purple dragons beyond the edge of the universe. Yeah, exactly. I mean, they don't even have to be that far from you. They just have to be not in your backward light cone.

So the idea is if general relativity is true, if Einstein's theory is true, that it itself implies that we can't know the global strong. structure of the universe because of the limitations that the theory places on how evidence must work, it has to be contained in your backward light. We have a case where no matter how much data you gather, you're going to have different possibilities for what the universe is like that the data doesn't distinguish between. Well, it seems to me in the end to come down to a form of skepticism to say like look there could always be some other theory out there that makes the same predictions but has a different story about the universe and so is the conclusion then i mean what does

that mean about our understanding of the universe as you say does that mean we shouldn't take too seriously the story about what's happening when we write Feynman diagrams they're a great little trick for making calculations but does that mean that we can't really believe that the universe is doing its own findment calculations, that fields are real because there could be schmields instead? Does that mean we can never really know what's happening out there in the universe? Yeah, so it all turns on how seriously you take this possibility of alternative theories that have the same empirical consequences as our current theories. So notice all we've done here is I've given a bunch of examples of pairs of theories where the evidence can't tell you

which one is right. I haven't given you a general procedure that you can use to construct from our past physical theory, a rival that agrees with it in all of its predictions, but it's different. So that would be the gold standard, right? If we could do that, then I would be very disturbed. We would have a mechanism for generating pairs of theories where the evidence doesn't tell us which one is right, but they're different. Then we're sunk. I don't know how to answer our questions about the unobservable anymore. But we don't have that. All we have is a bunch of examples. So what's the right conclusion to draws? We do have a lot of examples. I mean, I've talked to I don't know. It depends on how many you think is a lot. I think we've talked about four. Let me just make

sure I understand what you're saying. You're trying to be careful not overstate your conclusions. You're saying we have some examples that suggest that it might be possible to have two theories that explain the universe equally well, but have different stories. But we don't have a general proof that that's always possible. They don't have like a procedure that says, if you have a great theory, I can always use it to generate an alternative theory that works just as well. So you're saying there should be a little bit of skepticism about our skepticism. Yeah, that's right. Or we should be modest about our skepticism. Right. I was reading this article by Philip Kitchener and he says,

give us a rival explanation and we'll consider whether it is sufficiently serious to threaten our confidence, which is basically like saying, maybe this is a problem in theory, but right now it's not one that's facing us, so maybe it won't. Yeah, I think that's right. So now philosophers have attempted through the years to give kind of algorithmic procedures for generating rival theories that are equally compatible with our data. The kind of algorithmic procedures they give, though, result in theories that most folks would

not take seriously. So let me give one example. So you have your theory of the universe. Now, I have a rival theory, like we'll call it the when you turn your back theory. My theory says that the universe behaves exactly like your theory says it does when it's being observed. But when it's not being observed, it behaves in some completely other specific and compatible way. Our theories are equally compatible with the evidence. My theory says exactly the same thing about how the universe will behave when it's being observed as your theory does

by construction. But my theory is different from your theory. Okay, so I mean, like, take this over to the physics department and tell a physicist. I mean, you can tell me. No one would take this theory seriously, right? This is ad hoc. It's totally gerrymandered. It's not interesting. It's not a scientifically compelling theory, my theory, that is. And so it's like easy to dismiss this as not a genuine case of underdetermination. Yeah, that's right. That's not something I would take seriously. I mean, it's essentially equivalent in all the important bits, right?

It's using the same calculational machinery to make predictions for actual observations. It's just adding some bells and whistles to the non-observed side of things. It just doesn't really seem to me to count as a compelling. different explanation for what's happening in the universe. Yeah, and going back to something you said earlier, like, we have other means by which we can choose what theory to believe. So our theories are equally compatible with the evidence, but your theory is much simpler than mine, right?

Yours has a kind of elegance that mine lacks. So we might think that that gives us better reason to believe your theory than mine. And so is that the strongest argument against us? underdeterminism to say, like, look, we don't have a perfect example and we can't arbitrarily generate one. So we don't really know that this is a problem. Yeah, I would say there are two kinds of arguments against the problem of underdetermination. So one is this one that you just mentioned, like how common are genuine cases of underdetermination in order for this to be a troubling problem, they had better to be really common. The other route is the thing that we were just

talking about. So appealing to what folks will call theoretical virtues that are non-empirical. So simplicity or fruitfulness or elegance in order to decide between rival theories that are equally compatible with the evidence. So just because two theories are equally compatible with the evidence doesn't mean that we don't have better reason to believe this one than we have reason to believe this one. That opens a whole can of worms, though. These are philosophy questions. What are the non-empirical reasons for believing something? What are the reasons that are not exhausted by just looking at the evidence for believing one hypothesis over another? That's philosophy, right? Right. And you say that in a way that makes it sound like it's philosophy, it's not science.

But I wonder sometimes if we're too crisp about, you know, making a delineation between those two things, because, as you say, often we're using philosophy to make decisions in science. We prefer this theory to that theory because it's simpler. Those are choices influenced by our philosophy of science. We're doing that kind of stuff all the time. The thing I find fascinating is that most people in particle physics have very strong opinions about philosophical questions, but they also often think philosophical questions are a waste of time, you know? Like if I go around CERN and ask people like, do you think the top quark or the Higgs boson is real?

It's there when we're not looking or it's just a tool in our calculations. They're like, dude, we discovered the Higgs boson here. We know it's real. We found it. There's a Nobel Prize for it. You're crazy. You have to take them on a pretty long walk to get them to the place. We're like, okay, that's true.

It's not directly observed. And therefore, we don't really know if it's there. And there could be another explanation, et cetera, et cetera. All right. I want to hear more about that. But first, we have to take another quick break. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys.

Then, at 6.33 p.m., everything changed. There's been a bombing at the TWA terminal. Apparently, the explosion actually impelled metal, glass. The injured were being loaded into ambulance. is just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, and it was here to stay. Terrorism.

Law and Order Criminal Justice System is back. In Season 2, we're turning our focus to a threat that hides in plain sight. That's harder to predict and even harder to stop. Listen to the new season of Law and Order Criminal Justice System on the IHeart Radio app, Apple Podcasts, or wherever you can. get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Oh, wait a minute, Sam.

Maybe her boyfriend's just looking for extra credit. Well, Dakota, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now, he's insisting we get to know each other, but I just want her gone. Now, hold up. Isn't that against school policy? That sounds totally inappropriate.

Well, according to this person, this is her boyfriend's former professor, and they're the same age. And it's even more likely that they're cheating. He insists there's nothing between them. I mean, do you believe him? Well, he's certainly trying to get this person to believe him because he now wants them both to meet. So, do we find out if this person's boyfriend really cheated with his professor or not? To hear the explosive finale, listen to the OK Storytime podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast. I'm Dr. Joy Harden Bradford.

And in session 421 of therapy for black girls, sit down with Dr. Afea and Billy Shaka to explore how our hair connects to our identity, mental health, and the ways we heal. Because I think hair is a complex language system, right? In terms of it can tell how old you are, your marital status, where you're from, you're a spiritual belief. But I think with social media, there's like a hyperfixation and observation of our hair, right?

That this is sometimes the first thing someone sees when we make a post or a reel. It's how our hair is styled. You talk about the important role hairstylists play in our community, the pressure to always look put together, and how breaking up with perfection can actually free us. Plus, if you're someone who gets anxious about flying, don't miss session 418 with Dr. Angela Neil Barnett, where we dive into managing flight anxiety.

Listen to therapy for black girls on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast. Get fired up, y'all. Season two of Good Game with Sarah Spain is underway. We just welcomed one of my favorite people and an incomparable soccer icon, Megan Rapino, to the show, and we had a blast. We talked about her recent 40th birthday celebrations, co-hosting a podcast with her fiancé Sue Bird, watching former teammates retire and more. Never a dull moment with Pino. Take a listen. What do you miss the most about

being a pro athlete? The final. The final. And the locker room. I really, really, like, you just, you can't replicate. You can't get back. showing up to locker room every morning just to shi-talk. We've got more incredible guests like the legendary Candace Parker and college superstar A.Z. Fudd. I mean, seriously, y'all, the guest list is absolutely stacked for season two. And, you know, we're always going to keep you up to speed on all the news and happenings around the women's sports world as well. So make sure you listen to Good Game with Sarah Spain on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Presented by Capital One, founding partner of IHeart Women's Sports.

All right, this is Daniel, and we are doing an episode on underdeterminism in science, wondering whether it's possible to explain the universe in two different but equivalent ways. Well, my question to you is, what do you think are the prospects for making progress? Like, is this something we can ever really resolve? I mean, could it be that in a million years we're doing. science and we're still worrying about this kind of thing, like maybe somebody's going to come up with another theory that explains what's going on? Or do you think philosophers can, like, resolve questions like this that they can, you know, come up with a proof that two theories that give the same predictions are fundamentally categorically the same or that you could develop the kind

of algorithm you're talking about earlier to generate an alternative theory that's realistic from an existing theory? What do you think are the prospects for making progress on this question? One thing I think is important to say, it's like in terms of progress, like setting aside under-determination stuff, like science is always making progress, right? Taking the problem of determination seriously doesn't imply that it isn't. Like, progress was made when we moved from Newton's theory of gravity to Einstein's. Like our theories get more and more predictively successful, and that is definitely progress. So even if we still take the problem of under-determination seriously, science is making a kind of

progress. So what about making progress on the problem of underdetermination itself? I wish I knew. Note that one of the routes that we were talking about in responding to the problem, just in fact, all of them involve one doing like philosophy, taking seriously the kinds of questions that philosophers of science take seriously. We have to identify what the things. The theoretical virtues are. So like what the reasons are we have to believe one theory rather than another. And then we have to argue that these help us make good decisions, like in terms of which theory we should believe. We also have to, as you were mentioning, like come to some kind of agreement on when two theories are saying the same thing or when they're genuine rivals to one

another. And these are hard philosophy questions. So, I mean, going back to something that you said earlier, like, I think there's this famous Feynman quote. So he once said that philosophy of sciences is useful to science or to scientists as ornithology is to birds. Oh, man. I mean, it's a great quote. Makes me chuckle every time. But man, like, look, birds are doing some ornithology here, right? Or implicitly, at least, or we have to in order to make progress on this kind of stuff. Yeah, that's true. It's a great zinger, but I think it's not necessarily very productive. Well, let me make the question a little bit more sort of vivid and concrete. Let's imagine a hypothetical scenario. Say, far in the future, or maybe not that far in the future, aliens arrive,

and they are, you know, scientific, and they use mathematics, and they do physics, and they have also been pursuing the project of revealing the fundamental nature of the universe, matter and energy and particles and space and time and all that stuff. And we get to sit down to cross the table from them and compare notes. So here we have like a completely independent scientific tradition, but you know, fundamentally pointing at the same thing so we can avoid questions of like what aliens do science and could we communicate with them. What do you think the chances are that their theory is compatible with ours? That it's the same theory but, you know, written with different kind of squiggles dressed in different clothing, or the chances that they really

have come up with a completely different mechanism to describe the universe. What do you think the chances are in that scenario that our two theories are compatible or incompatible? Yeah, I don't know, man. Your guess is as good as mine. If I had to guess, I'd say it's pretty unlikely that anyone else does science in exactly the way we do it. you know, like so much of the way that we do science is grounded in accidental things about what

humans are like. You know, we're medium-sized. We travel pretty slowly. It's grounded in the kinds of things that we care about what we want to do with our science, how our particular perceptual apparatus works, like how, you know, our eyes work, stuff like that. And the way that we do science is grounded in accidental facts about history, too. Given all this, it's hard for me to imagine that anyone else would do it exactly in the way that we do. Well, that's one of the things that makes me excited to talk to aliens about science, because, you know, if they're doing science the same way we are, then, hey, we might learn some cool science.

And if they're not, we might learn some cool things about ourselves and the way that we explore the universe and the way that, like, being human colors how we are seeing the universe. It's so hard to sort of get out of our own heads. And that's like maybe one way to do it. Yeah, when you hear from them, let me know. I think when they do arrive, we should send the philosophers first before we send the physicists, you know, in case they're not so friendly. All right. Well, thanks very much for chatting with me about this really fascinating question in philosophy and in science and in philosophy and science.

Really enjoyed it. Thanks very much for your time. Yeah, thanks for having me on, Daniel. It was a pleasure. Thanks for listening and remember that Daniel and Jorge 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. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys.

Then, everything changed. There's been a bombing at the TWA terminal, just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, terrorism. Listen to the new season of Law and Order Criminal Justice System on the IHeart Radio app, Apple Podcast. or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Wait a minute, Sam.

Maybe her boyfriend's just looking for extra credit. Well, Dakota, luckily, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend's been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now he's insisting we get to know each other, but I just want her gone.

Hold up. Isn't that against school policy? That seems inappropriate. Maybe find out how it ends by listening to the OK Storytime podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Why are TSA rules so confusing? You got a hood of you. I'll take it all. I'm Manny. I'm Noah.

This is Devin. And we're best friends and journalists with a new podcast called No Such Thing, where we get to the bottom of questions like that. Why are you screaming? I can't expect what to do. Now, if the rule was the same, go off on me. I deserve it. You know, lock him up. Listen to No Such Thing on the Eye.

Heart Radio app, Apple Podcasts, or wherever you get your podcasts. No such thing. This is an IHeart podcast.