Daniel and Kelly’s Extraordinary Universe - Does physics have to be causal and local?

Episode Date: September 18, 2025

Daniel and Kelly talk to Sean Carroll about where some of the basic assumptions we make about the Universe come from, and whether we need them.See omnystudio.com/listener for privacy information.

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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 course. courageously told stories. Listen to Family Secrets Season 12 on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts.

It's important that we just reassure people that they're not alone, and there is help out there. The Good Stuff podcast, Season 2, 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. One Tribe, save my life. 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. The big picture task, the ultimate quest of physics, is to make sense of the universe, to sort through our amazing and bonkers experience and find as compact and simple an explanation as

possible for why what happens happens. We've made a lot of progress, but we've also struggled together. Do quantum particles really collapse together across space time? Why is gravity so difficult to unify with quantum theory? Why does time flow forward? Sometimes I wonder if the reason we feel stuck is that we started from the wrong place. What if there's a simple, basic assumption that we're making that has led us down the wrong path. It's happened to us before, like when we assumed that time flows the same way across the universe, or that we were at the center of the solar system, or that light needed a medium to propagate. When we removed the mistaken assumption, the veil was pulled from our eyes and a simpler, clearer, if weirder

explanation emerged. Could that be the cause of our current head scratching? Is there some requirement we've imposed on physics that's keeping us from seeing the simple explanation staring us in the face? Today on the podcast, we'll knock on the doors the most basic, intuitive, foundational concepts in physics and ask, do we really need them? We'll question locality. Do things have to be in the same place in order to interact? And its cousin, causality, do causes have to come before effects? Welcome to Daniel and Kelly's extraordinary counterintuitive universe. Hello, I'm Kelly Wienersmith.

I study parasites and space. And after today's conversation, I feel really lucky that locality and causality just make a lot more sense in biology. Hi, I'm Daniel. I'm a particle physicist, and I like to pretend sometimes to be a philosopher. Ooh. Yeah, I enjoy philosophy too. Well, one of the things I love about physics is that it butts right up against philosophical questions. We're asking about the deep fundamental nature of the universe. And so the philosophical questions are like immediate and obvious.

I hear a little bit less about like the philosophy of biology. You know, what does it mean to be a parasite, this sort of stuff? Well, I mean, we have plenty of arguments about that at the conferences we go to. But I feel like for us, philosophy, well, what's the thing? the difference between philosophy and ethics? Is ethics a subset of philosophy? Because we talk a lot about ethics associated with the biology stuff that we're working on. So I'm more used to tackling ethical questions than like, you know, what does causality even mean? Well, I think you put your finger on it because any time you get bogged down in a conversation by defining your terms,

that's when you know you're doing philosophy. Ah, there we go. All right. Well, we spend a glorious amount of time talking about how to define terms today, and we've got Sean Carroll to help us do that, and I love all of the stories that he tells. But before we jump into our interview with Sean, we should hear what our audience thinks about what it means to be local and causal. That's right. As usual, I went out there to our group of volunteers to see what they thought about these fundamental concepts in physics and philosophy. So think about it for a moment before you hear these answers. Do you think physics has to be local and causal? Here's what our listeners had to say.

Physics used to be local to me years ago when I deeded a woman whose father was a physicist slash yes chemist at Bell Labs, but it's not anymore. Observations and experiments at much larger scales and much smaller

scales seem to show that both of those concepts break down in the right circumstances. Most physicists assume causality. I don't think physics have to be local, but I think they have to be causal, to have causality. I think classical physics is local and causal, but I think quantum mechanics, with its

spontaneous random activity and its ability to have spooky action at a distance, breaks that a little bit. Physics has to be local and causal, at least in our quarter of the universe. Otherwise, Einstein's spooky action would have your lamp turn on before you hit the switch. Newton would drop his apple in reverse, and all hell would have broken loose yesterday. I would think physics has to be local in order to be testable so that results are always predictable, but I'm not sure what causal means, maybe cause and effect. I think it's really interesting to think of physics as not being local because the implications would be kind of crazy, and maybe it would explain some of the stuff that we don't really understand yet. How about it has to be at least as local as gravity and at least as causal as quantum decay?

What an interesting question. Does physics have to be local and or casual? Well, if it's a dating app, I was supposed to be local. And because I had a casual relationship with physics in high school, I really don't get the question. But I would say, isn't physics universal and cosmological? Maybe things change based on locality and conditions. I say yes, physics has to be local and causal. Love these answers, both insightful and hilarious, especially the person who misinterpreted causal to be casual. Are you sure you spelled it right in your email, Daniel? I am not sure, actually. Maybe that was the cause of that joke. Oh, yeah. See, most things in my world, there's a cause and effect relationship.

And there's no going back in time. Well, you know, this really is a fundamental issue in philosophy. We were just discussing on the Discord this morning. does the universe have to make sense? Does there have to be an explanation for everything? I think as humans, we are curious and we want to know answers to questions about the universe because we assume that there are answers, right? That there is a thing that is happening and we can figure it out somehow. And sometimes these discussions of like locality and causality tell me that the universe could be very, very different from the way that we assume that it is and in a way that

might never make sense to us. And you know, the more we learn about quantum mechanics, the more it breaks my brain, to be honest, but, but the more interesting questions I realize that there are left to ask, like questions about locality and causality at the quantum mechanic level. That is what I will be thinking about tonight when I'm trying to fall asleep. And I'm not sure if it's going to keep me up or make me fall asleep more quickly. So we'll see. And to dig deep into these topics, we invited an expert in physics and philosophy on the show and a friend of the podcast, Sean Carroll, who's not afraid to get bogged down defining terms. Yeah, things got heated between you two at a couple points. So let's jump in. All right. It's my pleasure to welcome to the podcast,

Sean Carroll. Sean is a theoretical physicist who's done an important work on the foundations of quantum mechanics, cosmology, and the arrow of time. He holds a joint appointment between physics and philosophy at Johns Hopkins. He's also a prolific science communications. He's also a prolific science communicator, the host of the Mindscape podcast, which I hardly recommend for its impressively deep dives, and author of several books such as the biggest ideas in the universe. Sean, welcome to the podcast. Thanks very much for having me. This is the first time I've been on a podcast hosted by two previous guests of my podcast. So I like how we are closing the triangle. This is exciting. We are accelerating towards the podcast singularity. That's right. Well, speaking of singularities,

Today, we're going to dive really deep on something that I've always wanted to understand better in physics. And I can't imagine a better person to ask hard questions about issues on the boundary of philosophy and physics. I mean, I know a lot of people who are physicists who are interested in philosophy. I know a lot of philosophers who do some physics as well. But I don't think I know anybody else who literally has an appointment in physics and in philosophy. So congratulations on straddling that barrier. Thanks. It was not easy to make it happen.

And, you know, as you know, academia, we love our little silos. And, I mean, there's plenty of people in universities who are cross-appointed between departments. But a humanities and a science thing getting together is really hard to pull off, yeah. And really awesome. I think it's awesome. I'm having a good time. So, you know, it makes me feel special. Well, I don't have a joint appointment philosophy.

I do have a courtesy appointment in the philosophy department, which is just because I showed up to enough philosophy seminars and asked awkward questions that they were. They were like, who are you? Who are you? And then they were like, oh, do you want a courtesy appointment? It gives you nothing. That's very courteous, yeah. Exactly. It's like they're encouraging you to keep coming back, though.

That's nice. Yeah, exactly. Yeah, yeah. Although I did learn there's a big difference between the two fields. In physics seminars, it's totally normal to interrupt with questions. You don't understand something? Raise your hand, speak up, start a discussion. If I give a physics seminar, I feel like it's a failure if nobody's interrupted to discuss

something. First time I asked a question in the middle of a philosophy seminar, everybody in the room turned to me with horror. It's like objecting in the middle of a wedding or something, you know, like, hold your question to the end, sir. But then also at a philosophy colloquium, you know, often they will let the speaker talk for an hour, take a five-minute break, and then come back for an hour of questions, which physicists would never do. Physicists would be like, what? Do you expect us to listen to what was happening in the seminar? What's going on here? All right. Well, let's dive into the topic for today. We're talking about locality and causality. And these, of course, are intermingled, but we're going to take them one at a time if we can. So let's start with locality. Sean, how do you interpret the concept of locality? What does locality mean in physics? It doesn't mean, like, get your donuts from the store around the corner. It kind of does. I mean, it's pretty close to that. It's the idea that the donuts aren't that far away, the donuts that you're actually going to want. You know, when I get donuts, I'm more likely to go to the place a couple blocks away than to the place 3,000 miles away. And fundamentally, that's because of locality. You know, locality comes in different ways in different stretches of physics. But the basic idea is that there's this thing called space. And indeed, we can promote it to space time, and we can talk about that too. But there are places where we're

located in the universe. And that kind of sounds, you know, obvious. So, yes, there are places where we're located in the universe, but it's not so obvious. It's a very strong claim that what the universe is made out of is space and things inside of space, right? Like, I have a location, you have a location, this electron has a location, et cetera. And then furthermore, that when these things bump into each other, this sort of, the basic idea of locality is there is space and things in it. The next level idea of locality is that when different things interact, they do so at the same point in space time or at least at neighboring points. And you might say, well, wait a minute, like the sun exerts a gravitational field on the earth,

even though it's very far away, but there is a field in between the sun and the earth. And it's more like the sun affects its gravitational field right at the sun, and that affects the gravitational field right next to that and you work your way up to what's going on here on Earth. But let me back you up because you said something which already blew my mind, which is that it's a strong claim essentially to say that locations exist. Before we get to locality, the requirements that things have to interact at the same location, you're saying it's a big idea that there is space and there are locations. And if that's a strong claim, like, what's the opposite? Like, are you suggesting it's possible to have a universe without locations? Or a

Physicists just making things complicated again. No, in fact, what I was just about to say is the thing about that version of locality I just said, that the world is made of things located in space, is that it's false. It is clearly not true because there's this thing called quantum mechanics. And quantum mechanics says that's not what the world is made out of. In, again, various different levels of precision. Even if you just have one electron, there's a wave function for the electron, and that wave function has some profile throughout space.

So there's no such thing as the point at which the electron is located. But you might say like, okay, but fine. There is something called the value of the electron's wave function at every point in space. It's kind of like the value of the electric field or the gravitational field. It's still a kind of locality, which is fine until you have two electrons. When you have two electrons, now there's not the wave function of electron one in the wave function of electron two, there's the wave function quantum mechanically

for the system of both electrons at once. And that's what the world is. And that's not things located in space. It's something much we're weirder than that. And so quantum mechanics, and I'm sure we'll get into it, but quantum mechanics just makes the world look really not local at all.

And then the question I would argue is, why does it kind of look local, right? You know, why do we get along so well thinking that the world looks local if quantum mechanics is trying to tell us something different? Does this question about what local means, you know, does it not make sense at the quantum level? But when you go past the quantum level, it totally makes sense? Basically, yes. But I will just like be, I got to be a stickler in this whole conversation, right? Because this is one of those things where we think we know what the words mean and we're digging deeply into what they mean.

And so we can't be beholden to our folk wisdom about what these words mean. So when you say like the quantum level and not the quantum level, I just want to remind everyone, everything is the quantum level. There's no non-quantum level. The world is quantum.

It's not like you get quantum when you look at things that are very small. It's the opposite way around. You get classical when you look at things that are very big. So classical mechanics a la Isaac Newton, etc., becomes a good approximation when things are big and ponderous and macroscopic and so forth. And that's the world in which, yes, you're completely right.

The world looks in the classical limit as if it's made of objects with locations bumping into other objects when they are at the same location. Okay. And before we get quantum, like Isaac Newton's theory of gravity is already non-local, right? We already got quantum, Daniel. It is too late. We've been quantum. whole time. Weren't you listening? Yes.

And I acknowledge that Newton is made of quantum objects. Yeah, exactly. He is quantum, but he didn't know about quantum. And when he was developing his theory of gravity, he invented a concept which had instantaneous communication across space and time. And so if people think about

non-localities, this new, fangled thing that emerged from quantum mechanics, isn't it sort of the old fangled thing that we sort of accepted for a while and then gave up and now we're returning to? We had to be very, very careful, once again, sorry for being so careful. But there are once again two different levels of locality that we have to distinguish between. One is just what I said, like the equations of physics are local means that you write down the things the world is made of and the whole equation is a function of position, right?

So in Newtonian gravity, you're right, that's not quite true. Like when Isaac Newton literally wrote down the inverse square law for gravity, the rule that says that the gravitational force is weaker when you're further away, stronger when you're close, by a factor of one over the distance squared. And he worried that this seemed non-local, that here's the Earth, the Earth is being pulled on by the sun and the moon and everything in the universe, instantaneously, because he's Isaac Newton. He doesn't know about relativity yet. It doesn't know about special relativity anyway. So there is absolute space, absolute time. And Newton in the Principia said, like, yeah, this bothers me the fact that somehow the information about the gravitational field of everything in the universe is conveyed to the Earth immediately. And he literally said, I'm going to leave this for future generations to sort out.

I don't like it. It bugs me. I'm not smart enough. I'm just Isaac Newton. You know, eventually we will figure it out. Now, there's a sort of little known thing that has. happened, but I think is really crucial. In circa 1800, Pierre Simone Laplace solved that problem. And no one ever gives them credit for this. But Laplace pointed out that you can write down Newtonian

gravity as a field theory. He invented the idea of the gravitational potential. And by a field theory, we just mean rather than just saying there's a force acting on the earth. And to know what the force is, you have to know what everything in the universe is doing. He says, there is something called the gravitational potential field, and it's a field, so it has a value at every point in space. And the equation that that field obeys is entirely local. What is happening to the field right here at some point only depends on what's happening at right next door points. And it's exactly like we said at the beginning that the sun creates a dimple in the gravitational field at where the sun is, and that pulls on the field next to it, and that pulls

on the field next to it, and it works its way out to the earth. So in that sense, because there is a local field theory description of Newtonian gravity, Newtonian gravity is entirely local. But is it still instantaneous? Well, you've skipped ahead to the world of relativity where there's a speed limit. So when relativity comes along, which is long after either Newton or Laplace, now we have an idea that even if the laws of physics are local in space time, which they are like Einstein's equations and Maxwell's equations and whatever, still, if something happens at a particular point in space time, the effects of that thing only ripple out slower than or at the speed of light, right? You do not affect things instantaneously far away.

And what that means is that we can have a better, stronger, more satisfying version of locality, which is that not only do the equations simply exist at every point in space or every point in space time, but that you're not being affected by things infinitely far away instantaneously. So we have ex post facto retrofitted our notion of locality to demand not instantaneous communication between two different points of space. For Newton, that would have been fine. In the post-Einstein world, that's no longer okay, because this is. speed of light is a fundamental limit. Okay, so the biologist who lives in the world where locality works fine, thank you very much.

So if I can sort of summarize what's happening, so locality is a problem because at the quantum level, it doesn't really work and you scale up, but it sounds like you were just saying that locality is fine when you're talking about things like the sun impacting the earth because you've got a field and you're connected every point along the way. And so is there a conversation then mostly about the quantum level now? Am I following? Well, there's lots of conversations. Okay.

There's definitely the distinction we were just drawing is between pre-relativity classical physics and post-relativity classical physics, where we have glommed onto the notion of locality, the idea that signals don't travel faster than light. So there cannot be any instantaneous communication across distances. You know, in Isaac Newton's world, I could send a signal to the Andromeda Galaxy instantaneously by taking a planet and shaking it a little bit, right? Its gravitational force, in principle, although not in practice, would change instantaneously in the Andromeda galaxy. And so is it local, you know, kind of, it's fading away, but still, strictly speaking, it's hard to wrap your brain around. But now in Einstein's universe, if you shake a planet, it's going to take you a million light years for that signal to get to the Andromeda. a galaxy. But still, that's all within the limit that we call the classical limit of quantum mechanics. And so one place where our notions of locality are challenged and need to be updated

is in the quantum realm. But by the way, another place that they're challenged and need to be updated is in things like biology. Oh, no. Yes, because, you know, in biology, what happens is most of the biological organisms that you and I know and love are moving very slow compared to the speed of light. Yep. So even though they're big and they're in the classical world and they bump into each other, when things do happen, they happen essentially instantaneously, right?

The signals can get to, you know, across the organism very, very fast. And so it can often be useful at that higher emergent level of biology to have both local things, like here's a little cell or there's a little bacterium or whatever, but also global things, right? Global variables. What is the pH of your solution that you're in? Or, you know, what is the gradient of nutrients? Or is the human being happy or sad? Like, that's not a local thing. Where's the happiness? Where's the sadness, right? So both at the microscopic quantum level and at the emergent, physics-e, higher levels, the strict notions of locality that we have in classical relativistic physics become a little shaky.

All right. So let's trace it historically. We start with these concepts where you can have instantaneous interaction across space and time. Then we get special relativity, which gives us a cool concept of locality. You can only be influenced by things in your past light cone and influence things in your future light cone because signals take time to propagate, which is still a cool. concept of locality means that like very much something interacts with something else which interacts with something else so you have to have like a chain of interactions or you know wiggle

a propagation of this interaction but what about general relativity is general relativity local in the same way that special relativity is general relativity is almost as local as special relativity is so special relativity came along in 1905 this is Einstein's idea that you can reconcile the non-existence of any preferred frame of reference in the universe. There's no ether or anything like that, with the fact that everyone thinks that the speed of light is the same. All you have to do, says Einstein, is give up your conventional notions of what space is and what time is. Oh, easy. And marry them together. Yeah, that's why he's Einstein, you know, marry them together to make

space time. And then 10 years later, in general relativity, he says, you know what? I forgot to tell you, but space time, this arena in which the game of physics is played has a life of its own. It's dynamical. It has a geometry. It can be curved. It can respond to matter and energy and their motion in the universe. They're a player. They're not just the arena in which the game is played. So that makes things much trickier when it comes to saying, you know, what depends on what. like the future of the universe, the future of the curvature of the universe, et cetera, in general relativity, in principle,

this is a deterministic consequence of what's happening in the universe right now. But there can be limitations on that. There can be weird global structures in the universe. You know, there can be closed time-like curves where the time-like trajectories that a person can travel on in a rocket ship can loop back on themselves.

Space can be, curled up on itself. Space can be a tourist. There can be extra dimensions of space that are very tiny and things like that. So the fundamental equations of general relativity are still 100% local, but there are global consequences of those equations that get things a little more subtle. Well, what about concepts like wormholes? I mean, if we're talking about locality, general relativity gives us more flexibility and what locality means because it changes which points of space are near each other, right? So in principle, you open a wormhole between our galaxy in Andromeda, now some part of Andromeda is local.

Is that concept of locality sort of preserved through the wormhole? There is absolutely a concept of locality that is preserved even in the presence of wormholes. Like if you say, if I take a limit where I look at creatures or points of light or, you know, particles or whatever that are small compared to the curvature and the topology of space time, everything is local, all of the equations are local, all the times. dynamics are local. But you're right. You can imagine, general relativity gives you this freedom to imagine, okay, I'm going to make a shortcut in space time. I'm going to construct a wormhole that attaches two different parts of space that I thought were far away. But if I go from point A

to point B through the wormhole, I'm going to say it doesn't take that long at all. They're actually much closer. And this was an idea that it was around a long time. Einstein, as usual, was one of the first people to talk about wormholes, John Wheeler gave them the name. We have enough time to tell this one very, very amusing story here. It was when Carl Sagan wrote this novel contact that wormholes became important in physics. Because Sagan wrote his book and he wanted to get his hero, Ellie, across the galaxy very, very quickly. And Sagan, you know, was a great scientist, but he was a planetary scientist. He was not a fundamental theoretical physicist. He didn't know his general relativity very well. So he had her fall into a black hole.

in the original draft of the novel. The good news is that Carl Sagan was close friends with Kip Thorne, who does know his general relativity very well, and had him read the draft, and Kip said, you can't have her fall into a black hole, she'll get spaghettified and smushed, and that's not what you want. What you want is for her to fall into a wormhole,

and that will get her across the galaxy in a very short period of time. And so that's what happens in the novel and in the movie. But then Kip was thinking about it, and he thought about it, and he said, look, you know, usually we say it's bad if you can travel faster in the speed of light because just change your reference frame and now it looks like you're traveling backward in time. That's one of the reasons why in conventional physics we say you probably can't travel faster in the speed of light. It enables time travel. And he thought about it and his students

in post talks and they chatted about it and they realized that's because you can travel backward in time if you have a wormhole. And he wrote a series of papers with his collaborators on building time machines using wormholes. So it's a perfect example of how locally everything looks local. In a small enough region of spacetime, everything looks local. But globally, in the presence of that wormhole, your conventional notions of locality are all messed up. General relativity is mine-bending. Well, I hope we are acting on you at a distance and you're having fun at whatever location you find yourself in.

And when we get back, we will dig more into locality. I'm Jorge Ramos. And I'm Paola Ramos. Together we're launching The Moment, a new podcast about what it means to live through a time, as uncertain as this one. We sit down with politicians. I would be the first immigrant mayor in generations, but 40% of New Yorkers were born outside of this country. Artists and activists, I mean, do you ever feel demoralized? I might personally lose hope.

individual might lose the faith, but there's an institution that doesn't lose faith. And that's what I believe in. To bring you death and analysis from a unique Latino perspective. There's not a single day that Paola and I don't call or text each other, sharing news and thoughts about what's happening in the country. This new podcast will be a way to make that ongoing intergenerational conversation public. Listen to The Moment with Jorge Ramos and Paola Ramos as part of the MyCultura podcast network on the iHeartRadio app, Apple Podcasts, or wherever you get your podcasts.

It may look different, but native culture is very alive. My name is Nicole Garcia, and on Burn Sage, Burn Bridges, we aim to explore that culture. It was a huge honor to become a television writer because it does feel oddly, like, very traditional. It feels like Bob Dylan going electric, that this is something we've been doing for a hundred years. You carry with you a sense of purpose and confidence. That's Sierra Teller Ornelis, who with Rutherford Falls became the first native showrunner in television history.

On the podcast, Burn Sage Burn Bridges, we explore her story, along with other Native stories, such as the creation of the first Native Comic-Con or the importance of reservation basketball. Every day, Native people are striving to keep traditions alive while navigating the modern world, influencing and bringing our culture into the mainstream. Listen to Burn Sage Burn Bridges 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 Foundation. 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 2 takes a deep look into One Tribe Foundation. a nonprofit 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 life 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. I 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 Podcasts, or wherever you get your podcast. Hey, it's your favorite Jersey girl, Gia Jude Ice. Welcome to Casual Chaos, where I share my story. This week, I'm sitting down with Vanderpump Rural Star, Sheena Shea. I don't really talk to either of them, if I'm being honest. There will be an occasional text, one way or the other, from me to Ariana. Maybe a happy birthday from Ariana to me.

I think the last time I talked to Tom It was like, congrats on America's Got Talent This is a combo you don't want to miss Listen to Casual Chaos on the IHeart Radio app Apple Podcasts or wherever you get your podcasts All right, we are talking to Sean Carroll By the concepts of locality and causality and I think it's time to dig into the thing we've been dancing around,

which is locality in quantum mechanics. So we've been talking about how you have to be the same location to have interactions, but now we have this concept in quantum mechanics of extended states. You can have particles that interact and that are entangled with each other, so their fates are connected, and yet they're moving apart from each other. And we know from Bell's experiments that if you interact with one, it can collapse the wave function of the other. How do we gel locality and quantum mechanics, or does quantum mechanics require us to give up on locality?

Is the universe fundamentally non-local? So the answer you're going to get to this question is the universe fundamentally non-local because of quantum mechanics will be completely different if you ask a philosopher and a physicist. And they don't even disagree with each other. They're just caring about different things. Okay. So, and this is the fundamental weirdness of quantum mechanics is that when we tell you, the rules for quantum mechanics, it's kind of clodgy and awkward, right? In classical mechanics, we say, here's a thing, like a planet or a electric field or something like that. And here are the equations that govern how that thing behaves. F-equals'

M-A for Newton's laws or Maxwell's equations or whatever, and that's it. That's your theory. That's all you need. In quantum mechanics, we say, here's a thing. We call the thing the wave function of an electron or of a field or whatever. Wave functions are the things. Here's an equation, just like classical mechanics. The equation in this case is the Schrodinger equation, but you can rewrite it in different forms, Feynman, path integrals, or another way of doing it, etc. But then we don't stop there, right? Like classically, we would have stopped. In quantum mechanics, we say, oh, but there's more rules, and the rules have to do with what happens when you measure or observe the system, right? You can measure certain quantities, and you can't

predict the outcome you're going to get. You can predict the probability of getting certain outcomes, and there are rules for how that happens. And when you make the measurement, the state, the wave function, changes dramatically in something we call the collapse of the wave function. So there's basically two sets of rules that apply under different circumstances. The set of rules, which if you want to be technical and impress your friends at parties, are called unitary evolution. That's the part where you're just obeying the Schrodinger equation and not being observed. And then there's the measurement process where you actually measure something. And so the measurement process in quantum mechanics, well, let me get it exactly right.

The state of things in quantum mechanics, the wave function, is just manifestly non-local. It just isn't local. It's not a thing with a value at every point in space and time. That's not what it is, and everyone knows that's not what it is. But what is the importance of that? Well, on the unitary evolution side, the laws of physics, the Schrodinger equation, or what have you, still look perfectly local as far as we can tell. Indeed, the whole discipline of quantum field theory is based on the idea that the thing that you're quantizing to make

your theory are fields that have values in space and time and only interact with each other at the points in space and time where they overlap with each other. So the particle physicists, bless their hearts, Daniel, that's what they care about. They care about that unitary evolution part and they know that at the end of the day they're going to measure it and the wave function is going to collapse. But who cares? They have a lot of work to do with Feynman diagrams and things like that, just calculating the unitary evolution. And we spent a lot of money getting those particles to the same location in space so that they do interact with each other. There's a picture right behind you on your Zoom background of exactly that happening. Yes.

Getting them to the same point really matters. So the physicists are like, locality is crucially important. What are you talking about? It's like the most important thing. It's what makes quantum field theory go. And the philosophers will say, like, who cares about all your integrals and your Feynman diagrams and whatever, I care about what the measurement process is about and what the implications of that are, because that's the part of quantum mechanics that is not very well understood and needs deeper explication. And guess what? It's wildly non-local. That is the implication of the EPR thought experiment from Einstein, Podolsky, and Rosen, who say that particles can be entangled, and the measurement outcome you get on one particle can instantaneously

effect. The allowed measurement now comes on another particle. And then John Bell comes along and makes that very rigorous. And again, if I have just a couple of minutes to tell an amusing story here. It was David Bone, who was a young assistant professor at Princeton the 1950s, who wrote a textbook on quantum mechanics. And he says in the textbook, he quotes a theorem John von Neumann, famous physicist, who says, you can't reproduce the predictions of quantum mechanics using what are called hidden variables, saying that there's a wave function, but also extra variables that are being pushed around. And Einstein reads Bohm's book and calls Bohm into his office, right?

Because he's also at the Institute of Rand Study right there in Princeton and says, this is wrong. Like, I speak German. Van Neumann's book was only in German. And so, like, none of the Americans knew what was in his book. They just quoted it because they thought he was a genius. And Einstein said, he made a mistake. He doesn't cover a lot of the important possibilities that you might want to consider here.

So you've not shown or you're not even given a good argument that you can't have hidden variable theories. So Boehm is inspired by this and he goes off and invents a hidden variable theory. The only way to make it work, though, is to make it non-local. So the dynamics of that theory are explicitly non-local. And then everyone ignores him because everyone ignores the foundations of quantum mechanics by this point in time. It's the 50s. And then in the 60s, it's discovered by John Bell, who reads Boe's papers, notices that it's non-local and wonders to himself, like, is there a way of doing this hidden variable

thing without being non-local? And he basically proves that the answer is no. You cannot reproduce the predictions of quantum mechanics without being non-local. And he hid that from his friends. He was working at CERN at the time. He didn't tell anybody because it was considered disreputable. But a couple of years ago, people won the Nobel Prize for testing his predictions and showing that they were correct. So it's non-local, but. we discussed earlier that these interactions can't happen faster than the speed of light, which Daniel has also, we've talked about on the show, right, that they can't communicate or whatever it is that they're doing faster than the speed of light. So what is happening then if it's

not local and it's not traveling faster than the speed of light? Daniel, do you know what's happening? Does anyone know what's happening? If you know what's happening, you win. You solve quantum mechanics. Colon mechanics has been around for 100 years. Just a couple of weeks ago, nature, the journal came out with a poll that they did of working physicists who care about quantum mechanics showing there's no consensus whatsoever about what's going on at the deep level. So, Kelly, you're just, you're right. Like, I mean, you're not right because you asked a question, but you're, you know, charmingly adorable about thinking that we have the answer to that question because this is exactly

what we don't have the answer to. And most working physicists, by the way, just, you know, use the strategy called denial to deal with this. Like, they just say, just, it works, okay? Like, don't bug me about this. I can make the predictions. The predictions come true. But it bugged Einstein.

That's why he wrote this EPR paper back in 1935. You know, he basically, it's very unfair to Einstein what I'm about to say because he had a much more sophisticated argument. But basically, he points this out. He says, I can get two particles. They're entangled. They're very, very far away. I observe one.

And apparently, the formalism is telling me that. instantly changes the state of the other particle very far away. I'm Einstein. I know that's not possible. You can't change things instantaneously very far away. What's up with that? And we still don't know what's up with that. And I read that Bell, you know, he interpreted his thought experiments and the later actual experiments to mean, as you say, that there is no local hidden variable, right? The particles are not carrying with them some details created at the moment of their entanglement, which actually determine the outcome, but that there is this loophole that you could have global hidden variables. So I think it's widely misunderstood that Bell's experiments tell you there are no hidden

variables. It just tells you there's no local hidden variables. And as you say, a global theory is possible. But tell me about the different interpretations of quantum mechanics. I mean, Kelly asks like the question, what is real, what is happening? And now essentially we feel like there is no local realism. But do the different interpretations of quantum mechanics tell different stories about how to accommodate this. I mean, I know we have like Bohemian mechanics where you have a pilot wave, which is explicitly non-local. You have like a global theory, as you say. But with the Copenhagen interpretation, is that a non-local theory? Or does Copenhagen essentially shrug away this question the way it does most of the important issues? Yeah, I mean, look, it really is

fascinating. I do encourage anyone with a little bit of quantum mechanics knowledge to actually read the original EPR paper, Einstein, Podolsky, and Rosen because, you know, Daniel can back me up on this, but we physicists don't read the original papers.

Like, that's, you know, we have a textbook and that tells us what's going on. But the original papers are fascinating because they don't know the answer, right? They're struggling with figuring out what's going on.

So Einstein and Podolsky and Rosen, but I get the impression that the ideas were mostly from Einstein. They didn't just say, look, there is this spooky action at a distance that bugs us. They were much more careful than that. They tried their best to construct an argument that says there should be what they called local elements of reality.

And I think that it was Bell who later called these things beables, beables in the sense of things that be, things that are, things that exist, right? Philosophers and they're creating phrases for things. John Bell is a car carrying physicist, I got to say. He's doing philosophy, though, when he invents a new meaning for the. the word B. He's doing philosophy. So he invents this word beable. And it's exactly what Einstein wanted to be the case. Einstein wanted it to be the fact that at every point in space time, there's a fact of the matter about what is physically going on in the universe. And as we said, quantum mechanics doesn't say that. Bell wanted, you know, to really understand this locality

issue. And so the way that Bome solves the problem is there are local beables. Like in Boe's theory, there are particles. You know, when you see at the LHC the track of a particle, what Boehm would say is you're not seeing the wave function. You're seeing the particle. The particles there in addition to the wave function. But the equation that the particle follows is non-local. It depends on what all the other particles everywhere else are doing, which is really weird. And so Bell asked this question, can you come up with any theory where everything is 100% local and none of the dynamics are local, none of the things are non-local. I hope I said that correctly both times. And he proves the answer is no.

So the different strategies for solving this just take very different points of view. In BOMI mechanics, despite the bullet, there's a non-local evolution rule. There are what are called objective collapse models of quantum mechanics where the wave function just suddenly changes all over space all at once. That's very non-local in its own. In Everettian quantum mechanics, in the many worlds theory, you kind of sidestep the question. One of the axioms, one of the assumptions of premises of Bell's theorem is that measurements have definite outcomes. When you measure the spin of a particle, it will either be spin up or spin down.

And whatever it says is, well, it's spin up in one universe and it's spin down in another universe. So that's not quite what Bell had in mind. So people have huge arguments over whether or not many worlds is local or not. The answer, of course, is it depends on your definitions. But that is one of the reasons to preserve that kind of dynamical locality that you might like many worlds. Copenhagen, I'm just, I'm going to like boycott any question about the Copenhagen interpretation from now on because it's giving it too much credit. It's not well defined. It's not a theory.

Like many worlds, BOMian mechanics, spontaneous collapse models, these are theories. They have equations and they make predictions. Copenhagen just won't answer certain questions, which I don't think we should reward it by taking it seriously. Can we give a little more information about what the Copenhagen stuff means for the biologists in the room? Yes, yes. Copenhagen is a city in Denmark. I got that, Sean. Full of people doing quantum mechanics.

And it was a great time. You know, it was, again, fascinating to read the original papers. We're here in 2025, it's the, it's been dubbed the year of quantum, the international year of quantum because exactly 100 years ago, the first papers came out by Heisenberg and Schrodinger, et cetera, setting up quantum mechanics. And we still understand it. But in like the 10 years after 1925, Niels Bohr and Werner Heisenberg and Wolfgang Pauley, who all were sort of either affiliated.

with or spent a lot of time at Bohr's Institute in Copenhagen promulgated this way of thinking about quantum mechanics. And it's exactly what I already said. It said that you have a wave function and it solves the Schrodinger equation when you're not looking at it. And then when you do look at it, it collapses and you get a probability. But the philosophical side of the Copenhagen interpretation is the claim that there's no such thing as what is happening when when you are not looking at the quantum system. And this is very explicit in Heisenberg's papers from 1925, and it's one of the reasons why it's very hard to understand these papers. Stephen Weinberg, who is one of the most brilliant physicists of the 20th century, he said, like, he tried very hard to read Heisenberg's papers, and he has no idea what was going on there.

But the big philosophical move was, stop asking about where the electron is. There is no such thing as where the electron is. There is only where you will see it when you measure it. And that's really the fundamental ethos of the Copenhagen interpretation. And by that, by itself, that's fine. But it leaves a whole bunch of questions unanswered. That's the bad part. Number one, what is a measurement?

What counts is doing a measurement? Can a video camera do a measurement? Do you have to be a conscious observer to do a measurement? What if you don't have good eyesight? Does that count as a measurement? Number two, the Copenhagen interpretation says the classical world exists and is real. You and I are not quantum things. Daniel made the joke earlier about Isaac Newton being made of quantum mechanical things.

Werner Heisenberg didn't think that. He thought that Isaac Newton was classical and that the things that you look at in a microscope are quantum. And there's literally an idea called the Heisenberg cut. and this is somehow in the space of all things happening in the world, there's one side of the cut where there's the quantum stuff and the other side of the cut or it's the classical stuff. And like, who invented that? Where does that go?

Like, no one knows what is going on with any of this. And so the Copenhagen interpretation is kind of just what we teach students in our quantum mechanics courses, but it's hilariously ill-defined. And as a starting point, as the kind of conjectural hypothesis that we throw out to do physics, It's great. It's amazing. It makes perfect sense. The weird thing is we've been pretending for 100 years that it's somehow a satisfactory final answer. I think you've been slightly unfair to the Copenhagen interpretation.

I could be much more unfair if you wanted me to. I could be much harsher. And I can't believe I'm in the situation of defending it. I mean, I agree that there's a fundamental issue with the heart of it, which is that they don't define the distinction between, you know, what causes a collapse and what doesn't. What's a classical object and what's a quantum object? Absolutely. And that's a fatal error. But, you know, there's these other issues of like, are you conscious or, you know, does a human do it or an eyeball do it or a video camera do it? I think those are maybe side issues. But I agree at the core of it, Copenhagen, is ill-defined. Things are heating up.

So I got to, wait, I got to bump in here. You got to interrupt because I just gave a talk at the American Association of Physics Teachers, where I was talking about the foundations of quantum mechanics. And for that talk, I made a slide in which I appealed to authority. So I'm going to quote some people who are much smarter than me talking about the Copenhagen interpretation. Albert Einstein says, the theory is apt to beguile us into error in our search for a uniform basis for physics, because in my belief, it is an incomplete representation of real things. Erwin Schrodinger says, I don't like it, and I'm sorry I ever had anything to do with it. Hugh Everett says, this is a philosophical monstrosity. And then Carl Popper, who invented falsification as the demarcation in science and non-science, says the Copenhagen interpretation is a mistaken and even vicious doctrine.

Okay. So if you think I'm being unfair, like the people who care about this a lot, I think are pretty hardcore that this is not acceptable. Well, I'm glad you didn't hold back. My own personal anecdote about Copenhagen is I got to spend a year at the Boer Institute. And when I got there, they gave me an office. and I noticed that the office next door to mine was quite different in that it had a bathtub in it. And I thought, why is there a bathtub in his office? That can't be good.

Nothing good can happen. I learned the story that in the old days when you had an institute, you lived at the institute the way like the president lives at the White House. And so there was an apartment. And then later, after Boer was no longer there, they were like, well, let's just turn these into offices. And so somebody got the office with the bathtub in it. Why didn't they take out the bathtub? Yeah, why didn't they take out the bathtub? It's Boer's bathtub.

He had important thoughts in that tub, you know? you can't just get rid of it. You're going to crawl in when you need to solve your next big problem? All right. So on that note, let's solve the next big problem. I want to talk about quantum gravity and the implications, but first, let's detour to causality. We've talked about locality a lot. But let's take a break.

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Okay, we're back and we're talking to physicist and philosopher. Sometimes when you measure him, he collapses into one state or the other about causality and locality in physics. So we started the conversation about locality with the definition, and we've already touched on some of the concepts of causality. But can you give us a crisp definition for what we mean by causality in physics? I'm glad you said it in physics, because just like locality,

causality is a notion that physicists and philosophers both talk about, but they mean entirely different things. So let's first just mention what someone who is neither would mean, right? You know, we talk about causes and effects. If you say, like, why were you late for work? Someone might say, well, because the traffic was unusually bad,

there was an accident, right? And that's a meaningful statement. There's a cause. There's an accident. There is a fact that there was traffic and that there's another cause. There was traffic. And there's another fact you're late for work. And there's a chain of causes and effects that ripples throughout all of our existence.

And this goes back to, you know, Aristotle thinking about this kind of thing. So it's very, very common that we take that kind of word that we use in everyday language and repurpose it for some rigorous idea in physics or philosophy. So what the physicists have done post-Einstein is to notice this feature of relativity that we've already talked about, that when you do something at some location, some event, and space and time, the implications, the effects of that doing something can only ripple forward in time and they can only ripple forward in time slower than the speed of light. So really, physicists by the word causality, what they usually mean is that signals travel slower than the speed of light or at the speed of light. Certainly no faster than the speed of light. So an alien shoots their death ray at us from Andromeda. They can't kill us today or tomorrow or yesterday.

They can only kill us in a million years or so. Unless they shot it a million years ago, right, yes. Which is not well defined in relativity. So, yeah, I'm not going to, I'm not going to, I am worried about the aliens. So, but there's already attention there in that physicist's way of thinking because there's a super important feature of the everyday notion of causality, which is that the cause always happens before the effect, right? You would not convince anyone by saying there was an accident this morning because I was going to be late for work, right? That's just not how causes and effects work. You're late for work because there was an accident, not the other way around.

But the fundamental laws of physics, whether it's relativity or quantum mechanics or anything, are time reversal invariant, are invariant going forward and backward in time. So really, when the physicists talk about causality, they kind of mean signals propagate to the future at or slower in the speed of light, but also they get to us from the past at or slower than the speed of light. Both are equally good features of what physicists call causality. And philosophers want to know, okay, but if that's true, why in the macroscopic world of our everyday existence, do we have this strong feeling that causes precede effects? And probably that has something to do with entropy in the arrow of time, and it's a whole long story. All right. So in physics, we have this concept that the past determines the future. Essentially, it's a statement on top of the existing laws of physics, which, as you say, are mostly invariant with respect to time.

that there's a directionality to time. Is that a compact way to understand causality in physics? It might be a little bit too compact. So let me be, let's just expand it out a little bit. You know, back in circa 1800 again, our hero, Pierre Simone Laplace, points out this existing feature of classical mechanics. You know, Newton invented classical mechanics, more or less in its modern form, in the 1600s.

It took a while for Laplace to realize the implication of Neville's. Newton's laws, which is that information about what is happening in the universe is conserved over time. So if you live in a purely Newtonian universe, purely classical, if you know what happens at any one moment of time, if you know that perfectly, you know the position and the velocity of every single atom in the universe, okay, then according to Laplace, the laws of physics determine everything that will happen in the future, 100% reliability. And it also determines everything that happens in the past because there is no distinction between past and future in Newtonian mechanics.

So that's what we mean by information being conserved from moment to moment in time. So this is already kind of different than our conventional notion of causality. The knowledge of what is happening in the present determines equally well the future and the past. And so to recover from that description, our folk wisdom about causes preceding effects, the fundamental laws of physics are not enough. You also need to put in some boundary conditions. And usually what we do is we say the early universe, the Big Bang, 14 billion years ago, had very, very special conditions. They were low entropy. They were a very, very tiny kind of configuration in the space of all possibilities.

And because we know that, because we know the conditions that were there in the early universe, we have a better handle on what things were like in the past than we do in the future. We say the past is fixed, right? We informally think that the past is just in the books. There's no decision I can make now that will change the past. But we think there's a decision I can make now that will change the future. How to reconcile that with Laplace, the answer is, you know, I don't know. the position of velocity of every atom in the universe. I have some incomplete macroscopic information,

and that might be enough to really fix what happened in the past, like I have a photograph or a video record of it. It is never enough to completely fix what happens in the future. I think that's a really subtle and underappreciated point that the present determines the past in the way you say it. In some sense, another way to think about it is that from the present, you can recover the past because there's a unique moment. Like the details of the universe now can only have come from a certain history of the past.

And so in that sense, you can recover it. It's not like if I change the present, the past changes, but there's a unique path which I can recover from knowing enough about the present. I don't know if you saw that. Not great show devs. They had this whole concept that try to see an image of the crucifixion, for example, by measuring the motion of air. particles over Malaysia or whatever.

Well, yes, so one of the reasons why that is a TV show and not reality is because, again, one cannot emphasize enough that you don't have information about position and velocity of every particle in the universe. If you wanted to, just to drive it home, if you wanted to recover something that was happening in an event 2,000 years ago, even in principle, you would need to know every that is going on in the universe now to within a 2,000 light year radius. Because there were photons emitted from the Earth back 2,000 years ago, and they've been moving away from you at the speed of light.

So if you don't have those photons, you cannot recover what was going on back there in the past. But at the same time, in principle, you could, except there's quantum mechanics that gets in the way of this. But classically, you could absolutely do this. So it's an interesting back and forth about the rigidity of the laws of physics. What's amazing to me is that we can make as much progress as we can, talking about both the past and the future, given such amazingly limited information about the state of the universe. So let's dig into the quantum mechanics of it.

I mean, you describe something like a clockwork universe where things are deterministic, and in principle, if you knew all the information, you could predict the future, et cetera. But we know that's not our universe. We know there is fuzziness. And we talk a lot in physics, especially in popular science,

about quantum fluctuations, which are treated as if they have no cause. You know, why is there a galaxy here? Oh, there was a fluctuation in the early universe. Why is there no galaxy there? Oh, there was a fluctuation. Do those fluctuations in quantum mechanics, do they really have no cause?

Is there nothing that determines them? Well, at the risk of, yet again, being careful and pedantic, we have to distinguish between not having a cause and not being determined. Those are slightly two different things. Of course, you're right, in the macroscopic world, when you make a quantum measurement, the outcomes are not determined by the quantum state of the universe. As far as we know, there is no hidden information that would determine them. In theories like BOMian mechanics, there literally is hidden information that does determine them.

So BOMI mechanics is 100% deterministic. But we literally have no access to that information. So, you know, what good is it to think that this is a determined event, even though we don't know, we cannot know what is the thing determining it. But also, you know, we're cheating a little bit because we're taking, we're borrowing this notion of causality that has kind of been handed down since Aristotle, the idea that, you know, for every event we can assign a cause, which was never really very fundamentally rigorous. Like, okay, I'm late for work, and I assign the cause to that, that there was traffic that morning.

But what about assigning the cause to that that space time is four-dimensional? Like, if it weren't, I wouldn't have been late for work. Like, there's a whole bunch of facts about the universe on which this result depends. And this is full employment for philosophers to figure out exactly what you mean by the cause. But in physics, we've sidestepped that question by replacing the ideas of cause and effect with a much more clear, rigorous framework, which is patterns, essentially different equations that tell you from one thing, another thing is going to follow, or maybe you have some discrete version of physics or whatever it is. But an if-then statement, you know, if this

happens, then that happens. It's not quite cause and effect, even though we speak that way sometimes. Like the example I like to use is the real numbers, right? Zero, one, two, three, minus one, minus two, minus three. There's a pattern there. If I tell you the number N, you can figure out what the number n plus one is. If I tell you five, you can figure out six. But five is not the cause of six, right? It's just the previous thing in the pattern. That's how the laws of physics work. It's one damn thing after another. And it's okay if some of those laws are stochastic, right? Like maybe they are, since we don't understand the foundations of quantum mechanics perfectly well. Classical mechanics was deterministic, but maybe quantum

mechanics just isn't, even at the most fundamental level. Or maybe it is, as I said, bomi mechanics, many worlds, these are deterministic theories, but we don't know which one, if either one of those is right. So I wouldn't say that quantum events don't have a cause. Quantum events follow the patterns given to us by the laws of physics. As to whether those patterns are deterministic or stochastic, we just don't know at the fundamental level. We do know that as observers in the universe, they seem stochastic to us. Right. And I didn't mean to imply that like anything goes in quantum mechanics, you do an experiment

and like anything can happen. Obviously, we construct conditions and quantum mechanics gives us predictions for probability distributions in that sense. It determines those distributions, but not the individual experiment. That's an important distinction. Thank you. No, it's actually super important because a lot of people, when you tell them that the way that fundamental physics works isn't exactly in line with our informal

2,500-year-old notion of cause and effect, they instantly leap to, oh, then anything goes. But that's really not what it is. There's still laws. There are still patterns. Maybe there's a stochastic element to them. We're just not sure. So then let me ask you to speculate wildly because we're peering into the future where maybe folks smarter than us are going to unravel the nature of space time and give us the theory of quantum

gravity that lets us understand all of this stuff. Do you think that theory is going to be non-local and causal only in the way the quantum mechanics is? Or do we have no idea about what's going to happen with quantum gravity? So your second guess is probably more accurate, more fair, more legitimate, more honest to say we have no idea what's going to happen. I have my favorite ideas. And my favorite idea is just taking quantum mechanics really super duper seriously. So what do I mean by that? I already said that when it comes to locality, the way that we represent the state of a physical system in quantum mechanics with a wave function or whatever is non-local, right? You have a wave function that depends on all the particles and all the fields, not on just one location in space.

You know, that's a sort of particular specific version of a more general abstract statement that quantum states are, you know, elements of some abstract mathematical space. And space, good old space, good old three-dimensional XYZ space, is not there in the fundamental description of quantum mechanics. Time is, which is weird, because the Schrodinger equation has a T in it, but the general former, the Schrodinger equation does not have an X in it. And that's in tension with the spirit of relativity. And that's just true and okay, we're going to deal with that, et cetera. But to me, the real deep question is not how do we reconcile ourselves to the apparent non-locality of

quantum measurement, the apparent spooky action at a distance when we measure one particle and we see an instantaneous effect on its quantum state? That's not the question. The question is, Because remember, there's this two sides of quantum mechanics. What happens when you're not looking at it where everything looks local and what happens when you look at it when there are these non-local correlations? I'm interested in why things ever look local at all. You know, if you just think that your starting point is this abstract quantum mechanical state vector of the universe, Hilbert Space is the mathematical space in which these quantum states live,

why does it look like we live in space and have approximately local interactions at all? And in fact, so I've written papers about this. Like you can actually be a working physicist and try to ask this question. It's a little bit hard to make too much progress because the question is so grandiose and we don't know a lot about it. But I think that it's a different angle on quantum gravity than the traditional ones. You know, traditionally in all of physics, what we do is. is we invent a classical theory of something, like electromagnetism or the simple harmonic oscillator,

or a propagating string in 10 dimensions. And all of these are classical descriptions. And then we quantize them. We have some rules for turning that quantum theory into a classical theory. And we keep bumping into problems when applying these rules to the questions of quantum gravity. But nature doesn't work that way.

Nature doesn't start with a classical theory and quantize it. Nature is just quantum from the start. So maybe the obstacle defining the right theory of quantum gravity is that we keep wanting to start with a classical theory in quantizing it. Maybe you should start with a quantum theory of nothing at all and asking under what conditions might it look like the classical three spatial dimensional universe with certain particles and fields and stuff like that. So I think that thinking about locality in this way might very well turn out to be absolutely crucial to making progress in quantum

gravity. So I think the description you're suggesting here is some concept in which space itself is not fundamental. It's emergent. Absolutely. And that's actually remarkably common among people who think about quantum gravity, that space itself is not fundamental. What's not really well understood is what that means. Like there's only one way to be fundamental and there's many ways to not be fundamental. So, okay, space is emergent. It's not fundamental. What does that mean? What is it? Where did it come from? And different people have different opinions about that. And it's a real struggle because we think geometrically. I mean, if you tell me, okay, the universe is a bunch of wave functions and they're entangled

together, I try to think, where are they? I imagine them in my head. Yeah. Because I think about where do I put it? And in my head, I have a space. And that space is three-dimensional, because I have lived in three-dimensional space. So I think in 3D. So it's a real struggle to counter our intuitions and to try to come up with a conception

of the universe that doesn't make these assumptions. So one question I have that is well beyond my pay grade, but could you model a virtual reality environment in which space is four-dimensional? Could you retrain your brain to move and live and perceive things in four-dimensional space? Or is there something in the structure of our brain itself that only make sense in three-dimensions? You know, people like Amani-Wa-Cont, the philosopher, you know, had an argument that three-dimensional space was kind of necessary. People argue about exactly how necessary he thought it was, but it was almost an anthropic

argument. You know, he tried to argue that there's the only way to make sense of the world as if you have this spatial arena in which things have locations and things like that. It's a, what is it, a cautionary tale because philosophers should be careful that their ideas won't be overthrown by later advances in physics. But I don't know. And I don't, I have ideas that are just very vague and hand-wavy about why space should have emerged in the first place. Maybe not really anthropic, but maybe something about locality is very

helpful if you just want complexity at all. Like if literally every particle in the universe could instantaneously affect every other particle, like how do you get through the day? Like how do you make a living in a world like that? I don't know. So I do think that these questions are very deep and certainly not understood, but maybe understandable. I have a suspicion about whether we could have four-dimensional VR. I mean, I remember trying to play video games with my teenager. And I played a lot of video games as a kid. But, you know, the controller was simple. There was A, B. You know, there was a little directional thing. You could jump on whatever. So then I'm trying to play Halo with with my teenager. And there's like a knob for the direction your gun is, a knob for the direction

you move, and a knob for the direction your head goes. And this kid is moving through essentially six-dimensional space, you know, and he's controlling it. It's totally intuitive to him. And I'm, I'm like, I have to put my, this finger on this joystick and that finger on that joystick. And, of course, he, you know, headshotted me instantaneously every single time. So I think the human brain is probably plastic enough to be able to adapt to those kind of environments. But not yours. That's funny. You don't look that old, Daniel.

It's kind of weird. But I guess, all right, we're learning. That's the Zoom filter. But I want to take our brains out universal and think, of course, about how aliens experience the universe. Do you think if we get to talk to aliens that they will. have gone through a similar trajectory, you know, where they imagine the universe is local and causal, and then they discover, oh, actually, at its foundations, these are just intuitive assumptions we're making, and the universe doesn't respect them. Or do you think it's possible

that they grew up natively to imagine a non-local universe? I think that it's, there's attention there, because I do think that everything is possible when you ask these possibility questions, but some things are easier to imagine than others. I do think the embodiedness of we beings in three-dimensional space is pretty natural to imagine as a universal feature, even of alien life, because the classical world is really helpful to making predictions about what's going to happen, and it's a very good approximation to the world. So I suspect that whatever trajectory of scientific understanding, the aliens take, it will

start with classical three-dimensional physics. and move on from there. But you know, if the aliens get really good at either VR or uploading into the matrix or whatever, maybe they're very used to thinking and perceiving things in different numbers of dimensions. Maybe that's just a switch they flip when they go in there. I remember one more completely amusing but irrelevant story. I was a science consultant for the movie Tron Legacy. Fun.

And you might remember Tron, those of us who are old enough. I remember Tron when it came out. It was, you know, one of the first early 80s Disney movies that had a lot of computer graphics in it, right? And Jeff Bridges was in it and it was, you know, it was not great cinema by any stretch, but it was fun. Deeply influential on Young Daniel as well. Well, it was greatish. It was a certain kind of great, yeah. But the sequel that they had later in the 2000s, Tron Lerner.

Legacy, and I think they've had another one, right? Or they're making it. So Tron Legacy was not as successful, cinematically, and part of it was, when you made Tron, computer graphics were terrible, right? Like, it was amazing, you could do it at all.

And you were like, oh, my God, this was so mind-blowing that you were in this thing that was obviously a bunch of people on scooters with neon taped to them, right? But, you know, they had some computer graphics going on. But by, you know, 2010, 10 or whatever, you can just make everything look perfectly realistic. And so they did.

So it looked perfectly realistic. Like inside the video game, things look like the real world. But my attitude was like, but we live in the real world. Who wants to see that? What we want to see is something that doesn't look anything like the real world because you can make any world you want. And they did not go down that road. But I think that that movie still remains to be seen where people like are literally

living in six-dimensional space and fighting their motorcycle battles accordingly. Tron Legacy would have done so much better if they took your advice. So many things in life that could be said about. All right. Well, thank you for joining us today on this element of our struggle to understand the real world and what is real about the world. I appreciate your thoughts and comments, Sean. Thanks very much for having me.

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