Does physics have to be causal and local?

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 https://omnystudio.com/listener for privacy information.

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

• 0:00Speaker 00

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• 2:00Daniel Whiteson?

  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 spacetime? 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?

• 2:42Daniel Whiteson?

  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?

• 3:27Daniel Whiteson?

  And its cousin, causality: do causes have to come before effects? Welcome to Daniel and Kelly's extraordinary, counterintuitive universe.

• 3:49Kelly Weinersmith?

  Hello, I'm Kelly Wiener-Smith. 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.

• 4:00Daniel Whiteson?

  Hi, I'm Daniel. I'm a particle physicist and I like to pretend sometimes to be a philosopher.

• 4:06Kelly Weinersmith?

  Ooh. Yeah, I enjoy philosophy, too.

• 4:09Daniel Whiteson?

  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.

• 4:28Kelly Weinersmith?

  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 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?

• 4:52Daniel Whiteson?

  Well, I think you put your finger on it because anytime you get bogged down in a conversation by defining your terms, that's when you know you're doing philosophy.

• 5:00Kelly Weinersmith?

  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.

• 5:19Daniel Whiteson?

  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.

• 5:38Speaker 05

  Physics used to be local to me years ago when I dated a woman whose father was a physicist slash yes chemist at Bell Labs, but it's not anymore.

• 5:50Speaker 06

  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.

• 6:00Speaker 07

  I don't think physics have to be local. But I think they have to be causal, to have causality.

• 6:09Speaker 08

  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.

• 6:23Speaker 09

  Physics has to be local and causal, at least in our corner 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.

• 6:36Speaker 10

  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.

• 6:48Speaker 11

  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.

• 6:57Speaker 12

  How about it has to be at least as local as gravity and at least as causal as quantum decay?

• 7:06Speaker 13

  What an interesting question. Does physics have to be local and/or causal? 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?

• 7:27Daniel Whiteson?

  Maybe things change based on locality and conditions.

• 7:31Speaker 14

  I say yes, physics has to be local and causal.

• 7:34Daniel Whiteson?

  Love these answers, both insightful and hilarious, especially the person who misinterpreted causal to be casual.

• 7:43Kelly Weinersmith?

  Are you sure you spelled it right in your email, Daniel?

• 7:46Daniel Whiteson?

  I am not sure.

• 7:49Daniel Whiteson?

  Maybe that was the cause of that joke.

• 7:51Kelly Weinersmith?

  Oh, yeah, see, see, most things in my world, there's a cause and effect relationship. And there's no going back in time.

• 7:59Daniel Whiteson?

  Well, you know, this really is a fundamental issue in philosophy. We were just discussing on the Discord this morning, like, 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.

• 8:33Kelly Weinersmith?

  And you know, the more we learn about quantum mechanics, the more, the more it breaks my brain, to be honest. But, 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.

• 8:56Daniel Whiteson?

  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.

• 9:07Kelly Weinersmith?

  Yeah, things got heated between you two at a couple points. So let's jump in. All right.

• 9:13Daniel Whiteson?

  It's my pleasure to welcome to the podcast Sean Carroll. Sean is a theoretical physicist who's done 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 communicator, the host of the Mindscape podcast, which I heartily recommend for its impressively deep dives, and author several books, such as The Biggest Ideas in the Universe. Sean, welcome to the podcast.

• 9:39Speaker 15

  Thanks very much for having me. This is the first time I have been on a podcast hosted by two previous guests of my podcast. So I like how we are closing the triangle.

• 9:49Kelly Weinersmith?

  This is exciting.

• 9:50Daniel Whiteson?

  We are accelerating towards the podcast singularity.

• 9:55Daniel Whiteson?

  Well, speaking of singularities, today we're gonna 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.

• 10:22Speaker 15

  Thanks. It was not easy to make it happen. 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.

• 10:39Kelly Weinersmith?

  And really awesome.

• 10:40Speaker 15

  I think it's awesome. I'm having a good time. So, you know, it makes me feel special.

• 10:44Daniel Whiteson?

  Well, I don't have a joint appointment in 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 like, who are you?

• 10:56Speaker 15

  Who are you?

• 10:56Daniel Whiteson?

  And then they were like, oh, do you want a courtesy appointment? It gives you nothing.

• 11:00Speaker 15

  That's very courteous.

• 11:01Daniel Whiteson?

  Exactly.

• 11:02Kelly Weinersmith?

  It's like they're encouraging you to keep coming back, though. That's nice.

• 11:06Speaker 15

  Yeah, exactly. Yeah, yeah.

• 11:08Daniel Whiteson?

  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 ejecting in the middle of a wedding or something, you know? Like, hold your question to the end, sir.

• 11:35Speaker 15

  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? You expected us to listen to what was happening in the seminar? What's going on here?

• 11:53Daniel Whiteson?

  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.

• 12:13Speaker 15

  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 of 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 spacetime 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.

• 12:52Speaker 15

  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, 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 spacetime, 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.

• 13:34Speaker 15

  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.

• 13:44Daniel Whiteson?

  But let me back you up because you said something which already blew my mind.

• 13:48Speaker 15

  Yeah.

• 13:48Daniel Whiteson?

  Which is that it's a strong claim essentially to say that locations exist. Before we get to locality, the requirement 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.

• 14:01Speaker 15

  Right.

• 14:02Daniel Whiteson?

  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?

• 14:09Kelly Weinersmith?

  Or a physicist just making things complicated?

• 14:12Speaker 15

  No, in fact, what I was, what 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. And, 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

• 14:54Speaker 15

  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 and 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 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?

• 15:36Speaker 15

  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?

• 15:42Kelly Weinersmith?

  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?

• 15:52Speaker 15

  Basically, yes, but the, I will just like be, I gotta be a stickler in this whole conversation, right?

• 15:58Kelly Weinersmith?

  Good, good.

• 15:59Speaker 15

  Because we're, 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.

• 16:18Speaker 15

  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 à 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.

• 16:53Daniel Whiteson?

  And before we get quantum, like Isaac Newton's theory of gravity is already non-local, right?

• 17:00Speaker 15

  We already got quantum, Daniel. It is too late. We've been quantum this whole time. Weren't you listening? Yes.

• 17:06Daniel Whiteson?

  And I acknowledge that Newton is made of quantum objects.

• 17:09Speaker 15

  Yeah, exactly.

• 17:10Daniel Whiteson?

  He is quantum, but he didn't know about quantum.

• 17:12Speaker 15

  He didn't know.

• 17:13Daniel Whiteson?

  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?

• 17:31Speaker 15

  We had to be very, very careful. Once again, sorry for being so careful, but

• 17:35Daniel Whiteson?

  Please be careful.

• 17:36Speaker 15

  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.

• 18:21Speaker 15

  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. He 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 gonna 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. Eventually, we will figure it out." Now, there's a sort of little-known thing that happened, but I think is really crucial.

• 19:01Speaker 15

  In circa 1800, Pierre-Simon Laplace solved that problem. And no one ever gives him 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 the 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.

• 19:46Speaker 15

  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 spacetime, which they are, like Einstein's equations and Maxwell's equations and whatever,

• 20:31Speaker 15

  still, if something happens at a particular point in spacetime, 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 spacetime, 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.

• 21:16Speaker 15

  For Newton, that would have been fine. For, in the post-Einstein world, that's no longer okay, because the speed of light is a fundamental limit.

• 21:23Kelly Weinersmith?

  Okay, so the biologist who lives in the world where locality works fine, thank you very much, is, is, 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 our conversation then mostly about the quantum level now? Am I following?

• 21:54Speaker 15

  Well, there's lots of conversations.

• 21:56Kelly Weinersmith?

  Okay.

• 21:57Speaker 15

  There's definitely, the distinction we were just drawing is between pre-relativity classical physics and post-relativity classical physics, where we have glommed on to 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.

• 22:41Speaker 15

  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 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 there are challenges 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

• 23:25Speaker 15

  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

• 24:08Speaker 15

  quantum level and at the emergent, non-physic-y, higher levels, the strict notions of locality that we have in classical relativistic physics become a little shaky.

• 24:20Daniel Whiteson?

  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 cooler 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. It means that like-

• 24:43Speaker 15

  Very much, yeah.

• 24:44Daniel Whiteson?

  Something interacts with something else, which interacts with something else. So, you have to have like a chain of interactions, or, you know, wiggle or propagation of this interaction. But what about general relativity? Is general relativity local in the same way

• 24:58Daniel Whiteson?

  That special relativity is.

• 24:59Speaker 15

  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 nonexistence 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.

• 25:25Kelly Weinersmith?

  Oh, easy.

• 25:26Speaker 15

  And marry them together. Yeah, that's why he's Einstein, you know? Marry them together to make spacetime. And then 10 years later, in general relativity, he says, you know what? I forgot to tell you, but spacetime, 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.

• 26:11Speaker 15

  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 or 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 torus. 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.

• 26:51Daniel Whiteson?

  Well, what about concepts like wormholes? I mean, if we're talking about locality, general relativity gives us more flexibility in 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 and Andromeda. Now, some part of Andromeda is local. Is that concept of locality sort of preserved through the wormhole?

• 27:13Speaker 15

  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 spacetime, everything is local, all of the equations are local, all the dynamics are local. But you're right, you can imagine, general relativity gives you this freedom to imagine, okay, I'm gonna make a shortcut in spacetime. I'm gonna 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 gonna say, it doesn't take that long at all, they're actually much closer.

• 27:58Speaker 15

  And this was an idea that has been 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.

• 28:43Speaker 15

  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 you know, he thought about it and he said, look, you know, usually we say it's bad if you can travel faster than 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 than the speed of light. It enables time travel. And he thought about it and his students and postdocs and they chatted about it and

• 29:24Speaker 15

  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. German relativity is mine, Ben.

• 29:49Kelly Weinersmith?

  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.

• 30:08Speaker 00

  Hey, it's Nile Rodgers, host of Before the Moment, a Lincoln original series. I'm driving across the country, meeting some of today's most fascinating artists. We're exploring the stories before the moment that changed everything. This week, I'm joined by rising star Sienna Spiro.

• 30:27Speaker 16

  I love the 60s. I feel really jealous that I wasn't alive for it. I'd be interested, like, knowing what that was like and seeing the way culture and art has progressed today. How do you find it?

• 30:37Speaker 00

  Wait a minute.

• 30:38Speaker 16

  Sorry, I have so many questions. Sorry.

• 30:41Speaker 00

  You're not nervous, are you?

• 30:42Speaker 16

  I am nervous.

• 30:43Speaker 00

  You are?

• 30:44Speaker 16

  I am. I feel like insecure about my experience. I know I shouldn't, but you know, I'm 20, I haven't really lived my life yet. And then sitting in front of somebody that has lived an enormous life is intimidating. And sometimes I feel like talking about myself is just not as interesting. So that's why I feel a bit nervous. I'll try and, I'll try and ignore it, then.

• 31:02Speaker 00

  Please, dude. Experience Before the Moment, a Lincoln original series on the iHeartRadio app, YouTube, or wherever you get your podcasts.

• 31:12Speaker 17

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• 31:56Speaker 17

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• 32:42Speaker 18

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• 33:23Daniel Whiteson?

  All right, we are talking to Sean Carroll about 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?

• 34:06Speaker 15

  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 cludgy and awkward, right? In classical mechanics, we say, here's a thing, like a planet or an electric field or something like that. And here are the equations that govern how that thing behaves. F equals ma 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.

• 34:49Speaker 15

  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 are another way of doing it, et cetera. 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.

• 35:34Speaker 15

  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?

• 36:17Speaker 15

  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's going to collapse. But who cares? They have a lot of work to do with Feynman diagrams and things like that, just calculating

• 37:02Speaker 15

  the unitary evolution.

• 37:03Daniel Whiteson?

  And we spend a lot of money getting those particles to the same location in space so that they do interact with each other.

• 37:08Speaker 15

  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,

• 37:50Speaker 15

  and the measurement outcome you get on one particle can instantaneously affect the allowed measurement outcomes 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. Oh, always. It was David Bohm, who was a young assistant professor at Princeton in the 1950s, who wrote a textbook on quantum mechanics. And he says in the textbook, he quotes a theorem by 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?

• 38:33Speaker 15

  Because he's also at the Institute for Grand Study, right there in Princeton, and says, "This is wrong. Like, I speak German." And von 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 I said, "He made a mistake. He doesn't cover a lot of the important possibilities that you might want to consider here. So you have not shown, or you're not even given a good argument, that you can't have hidden variable theories." So Bohm 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.

• 39:16Speaker 15

  And then in the 60s, it's discovered by John Bell, who reads Bohm'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.

• 39:38Speaker 15

  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.

• 39:50Kelly Weinersmith?

  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?

• 40:08Speaker 15

  Daniel, do you know what's happening? Does anyone know what's happening? Like, if you know what's happening, you win. You solve quantum mechanics. Quantum mechanics has been around for 100 years.

• 40:16Kelly Weinersmith?

  All right.

• 40:17Speaker 15

  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, you're, you're didn't, you're not right because you asked a question, but you're, you know, charmingly adorable about thinking that we had the answer to that question because this is exactly what we don't have the answer to. And, 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

• 41:02Speaker 15

  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 and we still don't know what's up with that.

• 41:34Daniel Whiteson?

  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

• 42:19Daniel Whiteson?

  to accommodate this? I mean, I know we have like Bohmian mechanics where you have a pilot wave, which is explicitly non-local, you have 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?

• 42:37Speaker 15

  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. Like, they were much more careful than that. They tried their best to construct an argument that says there should be what they called

• 43:22Speaker 15

  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?

• 43:33Daniel Whiteson?

  Philosophers and they're creating phrases for things.

• 43:37Speaker 15

  John Bell is a is a car-carrying physicist, I gotta say.

• 43:41Daniel Whiteson?

  He's doing philosophy, though, when he invents a new meaning for the word be.

• 43:44Speaker 15

  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 spacetime, 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 Bohm solves the problem is there are local beables. Like in Bohm's theory, there are particles. You know, when you see at the LHC, the track of a particle, what Bohm would say is you're not seeing the wave function. You're seeing the particle. The particle is there in addition to the wave function.

• 44:29Speaker 15

  But the equation that the particle follows is non-local. It depends on what all the other particles everywhere else are doing, which is, 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 or non-local. I should, 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 Bohmian mechanics, you just bite the bullet. There is 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.

• 45:12Speaker 15

  That's very non-local in its own. In Everreadyian 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 what Everett says is, well, it's spin up in one universe and the 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.

• 45:57Speaker 15

  Copenhagen, I'm just, I'm gonna, I'm gonna 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, Bohmian 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.

• 46:23Kelly Weinersmith?

  Can we give a little more information about what the Copenhagen stuff means for the biologist in the room?

• 46:28Speaker 15

  Yes, yes. Copenhagen is a city in Denmark and

• 46:33Kelly Weinersmith?

  I got that, Sean.

• 46:34Speaker 15

  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, it's been dubbed the year of quantum because 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 Pauli, 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.

• 47:14Speaker 15

  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 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. Steven Weinberg, who was one of the most brilliant physicists of the 20th century, said like, he tried very hard to read Heisenberg's papers and he has no idea what was going on there.

• 47:56Speaker 15

  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, that's the bad part. Number one, what is a measurement? What counts as 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.

• 48:41Speaker 15

  You and I are not quantum things. You know, 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 where 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.

• 49:21Speaker 15

  And as a, 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.

• 49:37Daniel Whiteson?

  I think you've been slightly unfair to the Copenhagen interpretation.

• 49:40Speaker 15

  I could be much more unfair if you wanted me to. I could be much harsher.

• 49:44Daniel Whiteson?

  And I can't believe I'm in the situation of defending it. I mean, I agree that there's a fundamental issue at 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, 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.

• 50:12Kelly Weinersmith?

  Things are heating up.

• 50:13Daniel Whiteson?

  Things are heating up.

• 50:14Speaker 15

  So I gotta wait, I gotta, I gotta bump in here. I gotta 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 appeal to a hardy. 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."

• 50:56Speaker 15

  And then Carl Popper, who invented falsification as the demarcation between science and non-science, says, "The Copenhagen interpretation is a mistaken and even vicious doctrine."

• 51:05Daniel Whiteson?

  Okay.

• 51:06Speaker 15

  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.

• 51:14Daniel Whiteson?

  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 this office?

• 51:31Speaker 15

  That can't be good. Nothing good can happen.

• 51:34Daniel Whiteson?

  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're like, well, let's just turn these into offices. And so somebody got the office with the bathtub in it.

• 51:48Kelly Weinersmith?

  Why didn't they take out the bathtub?

• 51:50Speaker 15

  Yeah, why didn't they take out the bathtub?

• 51:52Daniel Whiteson?

  He had important thoughts in that tub, you know? You can't just get rid of it.

• 51:55Kelly Weinersmith?

  You're gonna crawl in when you need to solve your next big problem?

• 51:58Daniel Whiteson?

  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. And when we come back, we're going to talk about causality.

• 52:23Speaker 00

  Hey, it's Nile Rodgers, host of Before the Moment, a Lincoln original series. I'm driving across the country, meeting some of today's most fascinating artists. We're exploring the stories before the moment that changed everything. This week, I'm joined by Miguel.

• 52:42Speaker 19

  That last year, finishing that record was the hardest experience of my life. To find your creativity and to believe in yourself, because somewhere in the midst of it,

• 52:53Speaker 19

  I stopped believing that my voice mattered and that I could find it again. That's a challenge. I was not feeling very confident creatively. Only, again, defiant in my belief that

• 53:11Speaker 19

  what this is about is express something honest. Say something honest.

• 53:18Speaker 00

  Experience Before the Moment, a Lincoln original series on the iHeartRadio app, YouTube, or wherever you get your podcasts.

• 53:27Speaker 17

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• 55:36Daniel Whiteson?

  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 a definition, and we've already touched on some of the concepts of causality. But could you give us a crisp definition for what we mean by causality in physics?

• 55:57Speaker 15

  I'm glad you say 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 like 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 an effect, 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.

• 56:41Speaker 15

  It's very, very common that we take the kind of word that we use in everyday language and repurpose it for some rigorous idea in physics or philosophy, etc. 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 in 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

• 57:26Speaker 15

  the speed of light.

• 57:27Daniel Whiteson?

  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.

• 57:36Speaker 15

  Unless they shot it a million years ago, right, yes. Which is not well-defined in relativity, so yeah, I'm not gonna, I'm worried about the aliens. So, but there's already a tension 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

• 58:18Speaker 15

  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 than 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 and the arrow of time, and it's a whole long story.

• 58:57Daniel Whiteson?

  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?

• 59:16Speaker 15

  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-Simon 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 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

• 59:58Speaker 15

  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,

• 1:00:43Speaker 15

  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.

• 1:01:27Speaker 15

  How to reconcile that with Laplace? The answer is, you know, I don't know the position and 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.

• 1:01:49Daniel Whiteson?

  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 at which I can recover from knowing enough about the present. I don't know if you saw that not great show Devs, where they had this whole concept that tried to see an image of the crucifixion, for example, by measuring, you know, the motion of air particles over Malaysia or whatever.

• 1:02:33Speaker 15

  Well, yes, so one of the reasons why that is a TV show and not a 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 everything 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.

• 1:03:17Speaker 15

  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.

• 1:03:37Daniel Whiteson?

  So let's dig into the quantum mechanics of it. I mean, you described 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 that 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?

• 1:04:11Speaker 15

  Well, I, 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 Bohmian mechanics, there literally is hidden information that does determine them. So Bohmian mechanics is 100% deterministic. But we literally have no access to that information. So what good is it to think that this is a determined event, even though we don't know,

• 1:04:54Speaker 15

  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 spacetime is four dimensional? Like, if it weren't, I wouldn't have been late for work. You know, I mean, like, there's a whole bunch of facts about the universe on which this result depends. And, and this is full employment for philosophers to figure out exactly what you mean by the cause.

• 1:05:38Speaker 15

  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 differential equations that tell you from one thing, another thing is going to follow. Or maybe there's, 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? 0, 1, 2, 3, minus 1, minus 2, minus 3. There's a pattern there. If I tell you the number n, you can figure out what the number n plus 1 is. If I tell you 5, you can figure out 6.

• 1:06:22Speaker 15

  But 5 is not the cause of 6, 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, Bohmian 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.

• 1:07:05Speaker 15

  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.

• 1:07:16Daniel Whiteson?

  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.

• 1:07:35Speaker 15

  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's still patterns. Maybe there's a stochastic element to them. We're just not sure.

• 1:07:58Daniel Whiteson?

  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 spacetime and give us their 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?

• 1:08:22Speaker 15

  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

• 1:09:03Speaker 15

  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 form of 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 have to deal with that, etc. But to me, the real deep question is not how do we reconcile ourselves to the apparent non-locality of quantum measurement?

• 1:09:52Speaker 15

  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.

• 1:10:37Speaker 15

  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 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.

• 1:11:21Speaker 15

  Nature is just quantum from the start. So maybe the obstacle to finding the right theory of quantum gravity is that we keep wanting to start with a classical theory and 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.

• 1:11:55Daniel Whiteson?

  So I think the description you're suggesting here is some concept in which space itself is not fundamental, it's emergent?

• 1:12:03Speaker 15

  Absolutely. And that's actually remarkably common.

• 1:12:07Speaker 15

  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.

• 1:12:25Daniel Whiteson?

  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.

• 1:12:39Speaker 15

  Yeah.

• 1:12:39Daniel Whiteson?

  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.

• 1:12:54Speaker 15

  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 makes sense in three dimensions? You know, people like Immanuel Kant, 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 this is the only way to make sense of the world is if you have this spatial arena in which things have locations and things like that.

• 1:13:38Speaker 15

  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 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.

• 1:14:20Daniel Whiteson?

  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 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

• 1:15:03Speaker 15

  But not yours.

• 1:15:03Daniel Whiteson?

  Definitely not mine.

• 1:15:04Speaker 15

  That's funny. You don't, you don't look that old, Daniel. It's kind of weird, but I guess, all right, we're learning.

• 1:15:09Daniel Whiteson?

  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?

• 1:15:39Speaker 15

  I think that it's, there's a tension 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, you know, the classical world is really helpful to, you know, 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, and move on from there. But you know, if the aliens

• 1:16:21Speaker 15

  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, when they go in there. I remember one more completely amusing but irrelevant story. I was a science consultant for the movie Tron Legacy. Oh, 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 eighties Disney movies that had a lot of computer graphics in it, right? Jeff Bridges was in it and it was, you know, it was not great cinema by any stretch, but it was fun.

• 1:17:02Daniel Whiteson?

  Deeply influential on young Daniel as well.

• 1:17:05Speaker 15

  Well, it was great-ish. It was a certain kind of great, yeah. But the sequel that they had later in the 2000s, Tron Legacy, and I think they've had another one, right? But, or they're making it. But 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 is 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 or whatever, you can just make everything look perfectly realistic. And so they did.

• 1:17:50Speaker 15

  So it looked perfectly realistic. 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? Like, 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, 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.

• 1:18:20Kelly Weinersmith?

  Tron Legacy would have done so much better if they took your advice.

• 1:18:23Speaker 15

  So many things in life that could be said about, yeah.

• 1:18:27Daniel Whiteson?

  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.

• 1:18:36Speaker 15

  Thanks very much for having me.

• 1:18:44Kelly Weinersmith?

  Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We would love to hear from you.

• 1:18:49Daniel Whiteson?

  We really would. We want to know what questions you have about this extraordinary universe.

• 1:18:55Kelly Weinersmith?

  We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you.

• 1:19:02Daniel Whiteson?

  We really mean it. We answer every message. Email us at [email protected].

• 1:19:08Kelly Weinersmith?

  Or you can find us on social media. We have accounts on X, Instagram, Blue Sky, and on all of those platforms, you can find us at D and K Universe.

• 1:19:18Daniel Whiteson?

  Don't be shy. Write to us.

• 1:19:21Speaker 17

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