Does physics have to be causal and local?

25:27There'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? But 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.

26:06They'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. There can be closed time-like curves or the time-like trajectories that a person can travel on in a rocket ship can loop back on themselves.

26:46Space 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. 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.

27:22Is 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 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 dynamics are local. But you're right. You can imagine – general relativity gives you this freedom to imagine, OK, 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.

28:03But 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 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. And 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 was a great scientist, but he was a planetary scientist.

28:39He 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.

29:11But then Kip was thinking about it and he thought about it and he said, look, 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 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.

29:47So it's a perfect example of how locally everything looks local. In a small enough region of space time, everything looks local. But globally, in the presence of that wormhole, your conventional notions of locality are all messed up. General relativity is mind-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.

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33:01All 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.

33:35How 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, OK? 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 kludgy and awkward, right? In classical mechanics, we say here's a thing, like a planet or an electric field or something like that.

34:15And 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. 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.

34:46In 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.

35:24That'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.

36:06Indeed, 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.

36:41And 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.

37:15And 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 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. 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 from 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.

38:07And Einstein reads Bohm's book and calls Bohm into his office, right, because he's also at the Institute for Van Study right there in Princeton, and says, this is wrong. Like, I speak German. 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 Einstein said this is – 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.

38:40The 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 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. And he hid that from his friends.

39:18He 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?

39:47Does 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. All right. 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 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.

40:22And 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, OK? 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. 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.

41:01I'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.

41:38It 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 Bohmian 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?

42:10Or 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. 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, I get the impression that the ideas were mostly from Einstein.

42:47They 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 card-carrying physicist, I got to say. He's doing philosophy, though, when he invents a new meaning for the word be. He's doing philosophy. So he invents this word beable, and it's exactly what Einstein wanted to be the case.

43:29Einstein 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 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. 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. But the equation that the particle follows is non-local.

44:10It 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, or 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, just bite 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.

44:50That'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, 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 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.

45:28But that is one of the reasons to preserve that kind of dynamical locality that you might like many worlds. Copenhagen, I'm going to 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. Can we give a little more information about what the Copenhagen stuff means for the biologists in the room?

46:06Yes, 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 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 don't 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.

46:52And 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 he tried very hard to read Heisenberg's papers and he has no idea what was going on there.

47:34But 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 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?

48:11Number two, the Copenhagen interpretation says the classical world exists and is real. 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 there's the classical stuff.

48:45And 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.

49:20I 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 at the heart of it, which is that they don't define the distinction between 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.

49:51Things are heating up. So I got to – wait. I got to bump in here. I 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 appeal 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.

50:29Hugh Everett says, this is a philosophical monstrosity. And then Karl Popper, who invented falsification as the demarcation between 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 Bohr 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.

51:07And I thought, why is there a bathtub in this 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 Bohr 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 did they think I was a bathtub? It's Boris' bathtub. He had important thoughts in that tub. You know, you can't just get rid of it.

51:34You'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. And when we come back, we're going to talk about causality.

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54:42Okay, 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 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?

55:19We 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 was an accident. There was 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 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, etc.

55:58So 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 the speed of light. So an alien shoots their death ray at us from Andromeda.

56:37They 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 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.

57:17You'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 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?

57:59And probably that has something to do with entropy and 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-Simon Laplace, points out this existing feature of classical mechanics.

58:37You 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 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.

59:17And 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.

1:00:02They 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? Plus, the answer is, you know, I don't know the position and velocity of every atom in the universe.

1:00:42I have some incomplete macroscopic information. And that might be enough to really fix what happened in the past. Like if 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.

1:01:20It's not like if I change the present, the past changes, but there's a unique path in 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 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 everything that is going on in the universe now to within a 2 ,000 light year radius.

1:02:13Because 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.

1:02:44So 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?

1:03:16Is 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 Bohmian mechanics, there literally is hidden information that does determine them. So Bohmian mechanics is 100 percent deterministic.

1:03:52But 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? 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, OK, 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?

1:04:31Like if it weren't, I wouldn't have been late for work. I mean, 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 differential 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.

1:05:08It'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 OK if some of those laws are stochastic, right? Like maybe they are since we don't understand the foundations of quantum mechanics perfectly well.

1:05:48Classical mechanics was deterministic, but maybe quantum mechanics just isn't, even at the most fundamental level. Or maybe it is. As I said, Bohme 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.

1:06:26You 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's still patterns.

1:07:02Maybe 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.

1:07:39I 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.

1:08:21And 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 OK. We have 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.

1:09:05That's not the question. The question is – because remember, there's 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. 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.

1:09:43Like 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. 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. All of these are classical descriptions. And then we quantize them. We have some rules for turning that quantum theory into a classical theory.

1:10:19And 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 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 a 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.

1:11:02So 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.

1:11:35I 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 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?

1:12:10Could 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? People like Immanuel Kant, the philosopher, 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. He tried to argue that 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. 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.

1:12:53But I don't know. And 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.

1:13:30I mean, I remember trying to play video games with my teenager. And I played a lot of video games as a kid. But the controller was simple. There was A, B. 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 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 like, I have to put this finger on this joystick and that finger on that joystick.

1:14:01And, of course, he, you know, head-shotted 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. Definitely not mine. 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?

1:14:41Or do you think it's possible that they grew up natively to imagine a non-local universe? I think that 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 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.

1:15:25But, 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. Oh, fun. And you might remember Tron, those of us who are old enough. I remember Tron when it came out. It was one of the first early 80s Disney movies that had a lot of computer graphics in it, right? Jeff Bridges was in it, and it was not great cinema by any stretch, but it was fun.

1:16:09Deeply influential on young Daniel as well. Well, it was great-ish. is 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? 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 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 they had some computer graphics going on.

1:16:49But by 2010 or whatever, you can just make everything look perfectly realistic. And so they did. 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? 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 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.

1:17:29So many things in life that could be said about. Alright, 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.

1:17:50Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We would love to hear from you. We really would. We want to know what questions you have about this extraordinary universe. We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you. We really mean it. We answer every message. Email us at questions at danielandkelly.org. 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 &K Universe. Don't be shy. Write to us. I'm U.S. Transportation Secretary Sean Duffy.

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