241 | Tim Maudlin on Locality, Hidden Variables, and Quantum Foundations

0:00:00.4 Sean Carroll: Hello, everyone, and welcome to the Mindscape Podcast. I'm your host, Sean Carroll. Today's podcast is one of those long-awaited episodes. People have been asking for this one for a long time. You know, it's funny. Sometimes, there are some possible guests who are just so obviously Mindscape guests, and I haven't had them on yet, and people are like, "Well, what's wrong? Why doesn't he have them on yet? They must be feuding! There must be beef!" or something like that. That's not usually the correct answer. It's usually just that I'd like to space them out. As I've said many times, like to have a variety, and that includes both people who I know very well and I'm familiar with their work and the people I've never heard of before I got to looking for podcast guests. So Tim Maudlin is someone I've known for a long time as a leader in the philosophy of physics, or what we sometimes call the foundations of physics, so studying not physics as a philosopher, but studying nature as a philosopher, but doing so in such a way that you're looking at the foundational questions of nature. You're asking the why questions, the deep questions. You're trying to be very careful. And Tim has done very important work in thinking about spacetime and the geometry of spacetime, the nature of time, and the arrow of time.

0:01:15.5 SC: I will point to... I will try to link in the show notes to a wonderful mock debate that was done in the Foundational Questions Institute between Tim and Julian Barbour. Julian Barbour famously has advocated that time does not exist. Tim has famously advocated that not only does time exist, but the arrow of time is fundamental, not just an emergent approximation because of statistical mechanics. And what FQxI does is it has the debaters flip sides. So Julian was arguing in favor of the existence of time, Tim was arguing against it, and they were both really, really good, I gotta say. They were both quite persuasive for the points of view they didn't actually agree with, as well as being quite amusing along the way. But today, we're actually gonna be talking about quantum mechanics. In some sense, this can be thought of as a sequel to the podcast I did a while back with David Albert talking about quantum mechanics. David and Tim are very good friends and have worked together for a long time. They don't agree on everything, 'cause no two philosophers agree on everything, but they are united in their skepticism about the Everett interpretation of quantum mechanics. And so with David on the podcast, even though I'm pro-Everett, I wanna give the voice to sensible voices on the other side, sensible points of view. And so David explained why he doesn't like the Everett interpretation. What we didn't get to, ran out of time, was what he does like.

0:02:43.0 SC: So today with Tim, we're gonna be talking mostly about Bohmian or de Broglie-Bohm versions of quantum mechanics, sometimes called hidden variable versions of quantum mechanics. I keep calling them hidden variable theories, as most people do, even though, as advocates like to point out, the variables are not hidden. The extra variables that you add to quantum mechanics to make a hidden variable theory are the ones that you observe when you actually make a measurement. And physicists have not really caught on to hidden variable theories; they're not very popular. But certain sets of people, some physicists, some mathematicians, some philosophers have kept the flame alive, and interestingly, as we talk about in the podcast, the Nobel Prize last year was given to tests of John Bell's famous theorems, his famous inequalities, which were very much prompted. Bell's exploration of these theorems and his proof of them was prompted by David Bohm's hidden variable theory, which Bell thought was the best-formulated known version of quantum mechanics; Bell thought we should be teaching it in textbooks. So it's a little weird that the people who really take these theories seriously get good results that later lead to Nobel Prizes, yet the theories themselves are not very popular within physics.

0:04:00.0 SC: Again, I could give my reasons for not being a fan of hidden variable theories, but they are... As Tim says about Everett, they are absolutely a serious attempt. It's something that you should think about if you care about these things. And so we'll give the sales pitch for why you should take these seriously, and we'll talk a little bit about foundations of physics more generally and even a little bit about the arrow of time. And the other thing that I wanna mention, because Tim reminded me of it at the very end of the podcast, is Tim is the director, and I think his title is Director, but anyway, he's the founder of something called the John Bell Institute for the Foundations of Physics. Tim has been very active, not only in doing foundations of physics but in advocating for the status of this subdiscipline within philosophy and physics. And the Bell Institute, which is located in Croatia and serves as a place to meet and talk about foundations of physics, is trying to raise funds to get a permanent home. So they literally have a GoFundMe, and you can visit the John Bell Institute homepage at johnbellinstitute.org. That will link you to a donate button. You can donate to the permanent home for the John Bell Institute. We're not there yet. We need a bunch more donations, but I think they just started, so it's early days. Maybe Mindscape listeners will kick in a little bit. And with that, let's go.

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0:05:39.2 SC: Tim Maudlin, welcome to the Mindscape Podcast.

0:05:41.1 Tim Maudlin: Thank you. Good to be here.

0:05:42.6 SC: So you've been a champion for a little while of something called the foundations of physics. So I wanna ask you two questions. One is, what is that for the listeners out there, but also, what's your sales pitch? Like, hypothetically, if you're talking to your colleagues in a physics department or philosophy department, how do you explain to them why foundations of physics is so important?

0:06:03.9 TM: Okay. Oddly enough, I don't have to sell it to the philosophers. It's a weird situation; you have to sell it to the physicists. But it's very simple. Really, foundations of physics, as the name suggests, is a branch of physics, namely the one that asks, "What are the most fundamental items? How do we understand the most fundamental physical structures and things that there are?" And that's only part of physics because you could do a condensed matter physics where you say, "I'm just worried about other states or the physics of stars," and you take for granted a lot of the micro stuff, or at least how to handle it, and then you're building up from there, but if you just dig and dig and dig and say at the very bottom, "What's the stuff that doesn't get explained in terms of other stuff?" That's the foundations. And so if you do that in physics, that's the foundations of physics. In philosophy, it's easy. I actually just gave a talk about this to the undergraduates at Rutgers because that is part of what philosophers call metaphysics or ontology, which is, again, just the question of what exists asked at the most generic level. And if you're a physicalist, you think everything that exists is somehow physical. If you're not, you might think there are other things that aren't physical, but everybody pretty much agrees there's some physical stuff.

0:07:27.2 SC: Not everybody, sadly, but yes.

0:07:28.1 TM: Not everybody, of course. Every time you say something, you can think of a counterexample, unfortunately. Now the curious thing, if I describe it that way as you say, but why should you have to sell this to the physicist? Why isn't this just part of the physics curriculum? And that has a very strange and disturbing sociological answer that really goes back to quantum mechanics because they didn't really understand when the quantum mechanical formalism was first developed in a way that was very predictively successful. They really didn't understand what they were talking about.

0:08:05.0 SC: They, the physicists?

0:08:05.9 TM: They, the physicists, the physicists who came up with the math. And you can come up with math and understand how to manipulate it to get predictions, but still not look at that math and be quite sure what the physics underlying it is. And those were very good questions, and they were asked very rigorously and vociferously early on by people like Einstein, people like Schrodinger, people who are very deeply involved even in the development of the theory. But at a certain point, another crew led by Bohr and, to some extent, Heisenberg kind of said, "Well, don't ask those questions," either because, for some deep, philosophical reason, they're not good questions, although they sound pretty good, or eventually, this morphed into what people call "Shut up and calculate," where you don't even explain; you just say, "Don't ask those questions."

0:09:01.2 SC: I mean, my impression would have been that Bohr and Heisenberg were more like, "We've told you the answer to those questions," and then when the Americans got hold of it, it was like, "Just don't even ask those questions."

0:09:10.3 TM: That's my take on it, was that it was... I mean, Bohr's background was in neo-Kantian philosophy. He actually said very, very, very mysterious things. And I would say, early on, the idea of the Copenhagen School was say some incomprehensible stuff and then calculate, and then when it came to America, they cut out the first step. [laughter]

0:09:33.6 SC: Good. But you said sociological reasons. I mean, I can imagine why... Well, don't let me imagine. What is, in your opinion, the sociological reason why physicists would stop asking what is going on at the fundamental nature of reality?

0:09:50.2 TM: So, and I'm speculating here: When they first started, everybody's familiar with this picture of the atom as a kind of planetary system with a nucleus, and then these electrons whizzing about it; we still kind of picture it that way. And that was sort of what was called the old quantum theory. Said that, and it said, "By the way, these electron orbits are restricted; they can't just orbit anywhere where a planet can. They're restricted to these particular little orbits. And then all they can do is jump between them, and when they jump between them, they'll either give off or absorb some light," and there was a bunch of explanation that you could get out of that basic picture. But if you pursue that picture, the natural question is, "But how do they jump? I mean, how does it get from here to there, and how does it orbit when it's in one of these orbits?" and questions like that. And they kinda beavered away at that and couldn't make much progress on it.

0:10:45.8 TM: And I think they eventually got to the point where they didn't wanna say, "Yes, those are good questions which we can't answer." And they were satisfied that they were calculating correctly, and in order not to say, "There's really deep questions we don't know the answer to," they said, "No, no, no, we've got it. We just have the mathematical formalism. The only reason you're puzzled is that you're trying to push some classical picture on the microscopic realm, and it doesn't belong there, and your confusion is arising from you having unjustified desires for comprehension."

[laughter]

0:11:26.3 SC: I don't know if I've ever mentioned this to you. I have talked to fellow physicists, and I've opined that, despite everything, I personally care about more than just correctly predicting the outcomes of experiments; I actually care about talking about reality. And many of them are very open and explicit that all they wanna do is predict the outcomes of experiments.

0:11:49.4 TM: Yeah, and I think if somebody had a lot of funding and a lot of time, it would be interesting to figure out at what point these people started saying that, because my experience is that young people going into physics care a lot about trying to understand. And they get very upset, and I know this 'cause I teach foundations of physics in the philosophy department, and I get physics students who say, "This is why I went into the field. You're talking about the stuff I was interested in." And I think, in most cases, I can't imagine an 18-year-old saying, "I just wanna calculate," right? So I think that basic curiosity about the world gets beaten out of them, and I really think it's part of a physics education, unfortunately, to beat it out of you.

0:12:38.1 SC: So we talked about Einstein and Bohr, etcetera, but it's still more or less true, you say?

0:12:43.7 TM: That's my impression. I certainly can testify that lots of physics students will come to me and say, "I get very frustrated. I raise questions to my physics professors of the form, "What's really going on here?" and I'm not told these are good questions. I really am told, "Don't ask them. Don't spend your time thinking about this."

0:13:06.7 SC: And slight generalization, but the philosophers, to their credit, have kept the flame alive. They've been thinking about these things.

0:13:13.7 TM: Yeah, yeah. You know, it's the same way that Aristotle kind of had to be squirreled away in the Islamic world for a while and then eventually found his way back. [laughter] Yes. I do think that that, really, philosophy has been a refuge. And in some cases, as you know, like David Albert got his degree in theoretical physics at Rockefeller, his background was not in philosophy, but he's in a philosophy department because these questions were welcomed in philosophy departments and not welcomed in physics departments. Now, my feeling, I have other people around us, and I have some concrete reason to believe things are getting better. I actually got an email kind of out of the blue from a chair of a physics department who said, "We're thinking of trying to introduce foundations of physics into our program. Can you give me advice... "

0:14:04.2 SC: Oh my goodness.

0:14:04.5 TM: "About what we might teach and how it might be done?" This was great, and this was somebody I'd never met, just, you know...

0:14:11.4 SC: I was literally just yesterday talking to an undergraduate thinking about grad school, and she wasn't sure... She likes the foundations of physics, but she's a physics undergrad, so she thought it'd be better to go to a physics department for grad school. Was asking for advice on where to go, and it was hard for me to even come up with departments who would let you do that right now, but maybe it's changed.

0:14:33.3 TM: Yeah, and at a certain time, I guess that you could say, "Well, you could go here because, yes, it's a normal physics department, but at least one or two people on the weekends and a little bit hiding from their colleagues are interested in these subjects and will talk to you about them if you catch them off their clock."

0:14:52.2 SC: Alright, so there's our sociology, but you already opened up the doors to what we wanna talk about, which is these mysteries of quantum mechanics, and David Albert was on the show. David Wallace was on the show; we were mostly talking about entropy and the arrow of time. Adam Becker gave us this wonderful historical introduction, but when we were talking to David Albert, mostly, we criticized the Everett interpretation of quantum mechanics. I say we [0:15:17.7] ____. I was defending it. I always let the guest talk, so David gave his spiel. What we didn't get time to get into is, okay, then what instead? So if... Let's not talk about Everett today unless it comes up. I wanna be substantive and proactive and constructive. What do we do if we're thinking about, "What are the mysteries we're trying to solve about quantum mechanics that got us into this trouble, and what are the avenues open to solving it?"

0:15:47.3 TM: Good, so... That's a really good question, and there are several different dimensions to it, but let me start with the dimension, and I am going to briefly bring in Everett but then let it go. Everybody knows there's this experiment; they say a thought experiment, although you could do it; it would be cool to do it, that Schrodinger talked about, with this cat penned up in a cage with a bit of radioactive material and a Geiger counter in this diabolical device. Actually, that turns out that Schrodinger was really just reconfiguring the same thought experiment that Einstein had written to him just before, which had to do with a keg of gunpowder that either would or would not explode in the next half hour, and you could calculate a quantum mechanical probability for it to explode and not explode.

0:16:08.3 TM: And what Schrodinger was pointing out and what Einstein was pointing out is that if you hold two views, which are very natural views you would like to hold, you got into what seemed to them to be hopeless difficulties, and the two views were, first of all, when we do quantum mechanics, we describe a system with a thing called a wave function mathematically. One of the questions is, "Okay, that wave function, does it provide us a complete description of the system, meaning every physical fact about that system, one way or another, could be extracted if you gave me the wave function?" And this was when Einstein, Podolsky, and Rosen wrote in 1935, and this is the same year as the cat paper, and the cat paper was really a response, or a reaction is better to say, to the EPR paper. The title of that paper is Quantum-Mechanical Description of Reality Complete. This is what they were looking at, and they were arguing in that paper, "No, there's more to the world than wave functions." If that were true, of course, then as a physicist, again, not as a philosopher, but as a physicist, the question is, "Okay, what else is there? Tell me about the other stuff."

0:18:00.4 SC: Mostly tests. [0:18:00.7] ____.

0:18:01.1 TM: So there's one question, "Is the wave function complete?" and then there's the question about how the wave function evolves through time, what its dynamics is, and Schrodinger wrote down this wonderful equation that has his name, that has some nice mathematical properties, particularly linearity. And people certainly wanted to say at least part of the dynamics is this Schrodinger evolution. And you might say, "Well, why not just go whole hog and say all of it is Schrodinger evolution? That's all it ever does." Okay? Now if you...

0:18:35.4 SC: I like that idea. That sounds promising. [laughter]

0:18:36.9 TM: Yeah. So if you put those two together and you say, A, the wave function is complete, and B, it always evolves by Schrodinger evolution, then you get into this problem that Schrodinger was pointing out and that Einstein was pointing out, which is that for the cat or for the gunpowder, after an hour, the wave function doesn't tell you either yes, it has exploded, or no, it has not exploded, or yes, the cat is dead, or no, the cat isn't dead. The wave function goes into this we call superposition of different states, which normally, you'd associate one state with the exploded gunpowder and another with the unexploded, but what the wave function gives you is not one or the other, but a kind of combination of both. And if you say, Well, that's a complete description, there isn't no further fact that's left out, then you say, but then the cat's neither alive nor dead or the gun powder somehow in a weird suspended state between exploded and not exploded and Schrodinger's view of that was that that was ridiculous. And John Bell, when he writes about this, I mean, the way he sums this up is he says, Either the wave function as given by the Schrodinger equation is not everything or it's not correct. And Everett... Now, I'm just gonna say the one thing about Everett is an attempt to say, no, it is both everything and correct.

0:20:01.1 SC: I came up with a way to make it both everything and correct.

0:20:03.0 TM: Right. So I would say that's the root that leads you to the Many-Worlds theory, and you understand the motivation, it's to keep both of those nice things, but it has this price to pay in terms of saying the idea that that one cat you put into the cage it either ended up alive or dead. No, that's not right. There are now, and not just two cats, actually infinitely many cats people don't point this out that there might be only one live cat, but there are gonna be a whole lot of dead cats 'cause the thing could have decayed at all these different times. Now, if you don't wanna go that way, and so we won't spend our time talking about the price you have to pay and what you have to do.

0:20:42.7 SC: I suppose, let's just get our cards on the table for the audience purposes... You are not a fan.

0:20:48.1 TM: I'm not, but it's a serious attempt. You have to be very clear here. There are things I'm not a fan of, but I appreciate the honesty of people who are fans, if they see what they have to confront and they straight forwardly say, "Yes. By making these commitments, I inherit these problems and I'm gonna work on those," right? And nobody should just turn their back, in my view, on a serious proposal. I take it to be a serious proposal, I think there are other proposals out there which I will not name right now, which I do not think are serious. I don't think that they have any chance, I would not put Many-Worlds in that basket.

0:21:28.8 SC: Gotcha, okay.

0:21:30.9 TM: But if we take off the table, having the wave function be both complete and always Schrodinger, then your options are, alright, if it's not complete, then there's something else in the world besides the wave function, and then immediately you ask, Okay, what is it? And what does it do? And why am I talking about wave functions anyway, and things like that, those are the natural questions you ought to have answers at your fingertips.

0:21:57.5 TM: Or if you say, Well, I think something went wrong with the Schrodinger evolution, then you can say, Well, I think the wave function doesn't always evolve in this way, it does this other thing that people call collapsing or reducing, that looks very, very, very un-Schrodinger like. This was the way that when John Von Neumann wrote down his understanding of the mathematical foundations of the theory had two processes for the wave function, the Schrodinger process and a collapse process, and that's also kind of okay, at a very high level, but as soon as you say that, you also now inherit an obligation to tell me, Alright, well, when and how does it deviate from Schrodinger evolution, and there are lots of details that can be put in in different ways and different suggestions. Some people think early on that the collapses had to be triggered by something, so things would go a nice Schrodinger-wise until X... What's the X?

0:23:00.9 TM: Oh, until a measurement is made or until an observation is made, and then that, of course, astonished... This is something Einstein was very upset about. Other people were upset about 'cause they say, Look, it's... What do you mean? An observation, who can do the observing? Can a mouse observe? Can a amoeba observe? Can they know trigger or collapse of the wave function? If you go that direction in a certain way, you end up with the Wigner who says, Oh, we have to talk about consciousness, and now you're in the mind body problem, and you know, you're in a mess. Other people have said, "Well, no, there's a kind of trigger or something that affects collapse, but it's not measurement or observation, it's like, say, gravity."

0:23:40.8 TM: This is something that Roger Penrose has been advocating for a long time, not with as much precision as a theory as some other things, but that somehow gravity, which is the one thing that is not well understood, quantum mechanically, may come in here as triggering this non-Schrodinger evolution. And then there's another idea, which he said, no it collapses and nothing triggers it, it just happens, it happens randomly from time to time at a certain rate, in a certain way, this is called the spontaneous collapse theories, and there's a famous theory by Ghirardi-Rimini-Weber, the GRW theory, and they just put numbers to it this was the great thing they did, they didn't just wave their hands, they said, Okay, yeah, there are collapses every now and then, how often? Well, once every 10 to 15 seconds per particle, and how does it collapse? Well, there you multiply by a Gaussian of this width and put some math to it. This is what John Bell was so impressed by. I know when I talked to him about that theory, was it okay for many years, people who have had this kind of general thought, but they took the step of turning it into clean mathematics.

0:24:56.9 SC: And also I think mathematics, but a real theory. Physically, right? Yeah, not just an idea. Not just a general suggestion.

0:25:04.6 TM: Yes, that's right.

0:25:05.3 SC: And they can be in fact experimentally tested.

0:25:06.0 TM: Yes, it does, it makes... Because the wavefunction evolution is different, and because the predictions really depend on what the wave function does, it will make... It's easy to see, in principle, it makes different predictions, it's harder to come up with the experimental situations where you can see those differences, although people have been working at it for some decades and making progress, the progress is mostly ruling out the theory. So far, we have not seen a signature that the theory is correct. Okay, we haven't ruled it out, but you could have ruled it in, if you'd seen certain things, you could have really ruled it in, it has not been ruled in.

0:25:42.4 SC: And ruling it in these spontaneous collapse theories would count as falsifying these other theories.

0:25:48.3 TM: That's right. If that were to happen than the other things on the table that we might talk about, including Everett, a lot of the motivation for Everett would go away because one of the motivations is... But gee the Schrodinger evolution is so nice. Right?

0:26:02.8 SC: So that is a motivation, I guess, to a physicist, There's a sort of... I don't wanna say intuitive, but emotional objection to these spontaneous collapse theories, like it just looks so ad hoc that the wave function just does this, and I guess you could say, yeah, but we're fitting the data.

0:26:21.0 TM: Yeah, and ad hoc. It's not that I don't have some sympathy, but when you realize how difficult these questions are and the weird extremes that people have gone to say, "Oh, but I don't like this one, 'cause it's ad hoc," that is such a weak tea. Right? I mean, get over it. The physical world might look a little more ad hoc than you prefer, you really shouldn't rule things out in a strong way for that reason.

0:26:51.7 SC: I guess maybe following what you said about Einstein and EPR, the one thing that you can say about quantum mechanics is that someone's gonna have to give up on something that they really want to be true.

0:27:02.0 TM: Yes, I think that's correct. I mean, yes someone is... I'm not sure I would say and now I'm gonna tip the hand, which you'd understand already, if you already know, I'm not that sympathetic to the Many-Worlds because of the problems I see there, and I also have this feeling that I can only say by saying the collapses don't smell right to me. I don't wanna put too much weight on that, but that's just the way I feel and when you're in this, you kind of make a judgment about what looks more plausible to you, then we have the last possibility where, "Okay, yeah, there no collapses, I agree. I like the Schrodinger evolution. It looks, it's clean, it's pretty. It's linear. I wanna keep that, but I want my cats to just... One cat goes in and one cat comes out," that means I have to have something in addition to the wave function, and now the question is, what? And the amazing thing, and I think anybody, whatever you think about the theories at least just for the non-relativistic theory, there's some complications further on. For the non-relativistic theory, this thought occurred immediately in 1926 already by Louis de Broglie.

0:28:15.0 TM: And you say, "What is this magical thing you're gonna add to your physics?" answer, particles, point particles. Just the sort of thing you were already familiar with, the sort of thing, even Democritus, while his were little thick, they had that shape, so these can be point particles, so they're even slim down and more minimalistic than Democritean particles, but the idea, Okay, there's some particles and they move around and they constitute cats and tables and chairs, and if you wanna know whether your cat's alive or dead, tell me where it's... What its particles are doing and I'll give you a good idea. You might say that's such a simple idea. It couldn't possibly be the case that you just add some particles and write down a pretty simple looking equation for how they move, and everything comes out right, but amazingly, in the non-relativistic theory, it does. I don't really see that as giving up on anything, because what you have to give up on is the completeness of the wave function, but it's not clear what the motivation for believing that ever was except Bohr so on saying, Look, we know what to do with the wave function, if it's complete, then we're done, and we wanna be done, so don't tell us there's something else.

0:29:29.5 SC: So in other words, the basic idea is when we invented quantum mechanics, we started saying, electrons are like waves, they have interference patterns under the right circumstances, they spread out in the atom, they also like particles in certain ways, we measure the little dots on screens, and the brilliant breakthrough from de Broglie is "That's because they're both at the same time."

0:29:52.1 TM: Yeah, or to put it as Bell again, I'm always quoting John Bell, he says people were breaking their heads all the time, wave or particle wave or particle, you'd hear wavicle, it's a new concept you don't have. And he said, "Why didn't it occur to them the simple answer, wave and particle." There is a wave that is a thing that obeys a wave equation, which is why you get interference, that's the wave function or the quantum state, the thing described by the wave function, and in addition, there is a particle which always has a location. So if you ask, but why do these little individual spots form on my screen, it's because that's where the particle hit. No reason why you can't explain the wave-like behavior because there is a wave and the particle-like behavior because there is a particle, and their dynamics is coupled in a way that the particles are guided by a wave-like thing, and so the interference in the wave will affect where the particles go.

0:30:49.2 SC: So this table in front of us is in some legitimate ends made of particles, and the positions of those particles are among other things, being guided by the wave function.

0:31:00.0 TM: Yes.

0:31:02.3 SC: Are they separately interacting with each other, the particles?

0:31:04.2 TM: Well, of course, the word interaction here is one of those words that different physical theories will give you different accounts of what even counts as an interaction. Certainly, the particles can be in a situation where what happens to one will make a difference to what happens to others, right, so the theory will just tell you, gee, you turn on a magnetic field over here and now this particle gets deflected down and by the way, that particle will get deflected up, the example I just gave you is a little bit misleading, but now, is that an interaction between the particles? Well, it's certainly mediated by the wave function, the particles can't do anything to each other, but in a way, already you have that in Newton you say, Well, how does one particle affect another gravitationally. Well, not sort of directly, but you need a gravitational field or something like that, there's a mediator, there's a physical mediator that accounts for these relations between what they do.

0:32:10.0 TM: Is that interaction? It is, but it's not quite like classical Newtonian interaction. One way to talk about this, people get a little bit... Maybe this will help, or maybe not, but let me say it briefly, if you've studied any physics, if you did any Newtonian physics, you'll remember that the fundamental equation is F = ma, where a is the acceleration, and the acceleration is the change in the velocity and the velocity is already the change in the position, so it's a second time derivative, and what that means for Newton, is it to give you the initial conditions of a system, I both have to tell you where everything is and what its momentum is, right, so I need both position and momentum, and then the fundamental kind of physical interaction through F = ma is changing the momentum.

0:33:02.3 TM: But in this theory, it's not like that, it's a first order theory, everything is just done with one-time derivative, so the initial condition only has to be the positions, and then what the wave function does is guide the particles, it doesn't push them around by producing Newtonian forces on them, it guides them as it were, determines where they go more in the way, this is a kind of analogy that David Bohm used to use as if you were piloting by radio control a little boat out on the lake, and you say, Yeah as you turn the dial, as it were sending information or telling the boat where to go, but you're not pushing the boat around, and in fact, if the boat goes twice as far away, so your radio signal is half as strong, still as long as it's getting the signal, it's gonna guide it the same way, right, whereas a Newtonian force, you'd sort of think, Well, the further away you get, the weaker the force has to become, so it's a different fundamental picture of physical interaction.

0:34:11.1 SC: And I think many physicists object to it or had this intuitive emotional response to it, because the wave function is guiding the particles, but the particles don't influence the wave function at all, maybe part of that negative feeling is that they're still thinking of the wave function as a Newtonian force field.

0:34:30.6 TM: They might be thinking that. This is an objection people have raised quite explicitly, that this theory violates they call the generalized Newton's third law of action and reaction. So they say, Oh well, in general, I think if A has an influence on B, B should have a back influence or a reciprocal influence back on A, and that I have to say strikes me as just silly. In a certain way, if you say to Newton, "Look, doesn't the law of gravity somehow affect what particles do?" He'd say, "Yeah, yeah, yeah." And you say, "But what of the particles? Do they affect the law of gravity?" He says, "No, of course not. The law of gravity doesn't change because of what the particles do. The way the particles behave is accounted for as it were by the law." So we don't usually think that if A affects B, B has to have a reciprocal back effect on A. Lots of cases where even in physics we wouldn't say that at all. So this seems to me to be a bit special pleading of trying to find something to complain about.

0:35:31.3 SC: I mean, certainly, it sounds like a very made up metaphysical principle. That they just made up. Okay, you mentioned David Bohm, tell us his role in this story.

0:35:39.0 TM: So it's an interesting... The history of this is very interesting because again, people were very concerned, there was the early quantum theory by Bohr, which had this very classical looking idea of this planetary atom and then these jumping kind of jumps which were unusual and they didn't quite understand. And then the new quantum theory was developed in the '20s, mid '20s, and the mathematics of it came out first, Matrix Mechanics by Heisenberg and wave mechanics by Schrodinger. I'm just throwing some names out, but people have heard these names. But they had the math in a way, they kinda knew how to manipulate the math to make some predictions, but it wasn't at all clear what the physical picture was, what's the physical theory here, and de Broglie early on looks at this and says, Well, I can just... I can just put these particles in these point particles and have them guided around by this quite simple equation that... You need a second equation, 'cause we needed an equation to tell us what the wave function does, and we're just gonna keep Schrodinger's equation for that. So we got that.

0:36:42.8 TM: No real dispute about that. Then we need a second one which tells me what the particles do and that's gonna be a thing called the Guidance equation, and just from mathematical simplicity and other very basic physical considerations, I think I've seen claimed 10 different ways that would lead you in the non-relativistic theory, to a very simple, obvious equation, guidance equation for this, lots of ways to motivate it intuitively.

0:37:12.4 TM: And so, de Broglie discovers this and he presents it, he was very young at the time, and some rather bad objections were made to the theory, but de Broglie on his feet couldn't answer them, and the story seems to be that he was just a bit traumatized by the whole thing. 'cause these were the big shots of physics. He's a young guy, he's at the Solvay conference with the biggest names, Einstein is there, Bohr is there and so on, and they're [0:37:38.9] ____ beating up on him, and he just kind of seems to have abandoned the theory a bit, as far as I can tell, and I've heard people tell me this more recently, who looked into it, it kind of lays fallow, now Einstein who we know didn't like the Copenhagen understanding of quantum theory we know, played around with this theory or very similar theories in the intervening years. He didn't like it, but he didn't like it 'cause it had what he called in a very manifest, easy to see way, spooky action at a distance, this non-locality, so we know why he didn't like the theory, and he could never come up with a satisfactory version to him. And then...

0:38:19.9 SC: But it's very possible that he came up with the version that other people think is satisfactory.

0:38:23.2 TM: Exactly. He could've, he could've very well had it, and it sort of stayed that way. And and then the really interesting situation if we get into this is David Bohm is now a young physicist, very people very well regarded at Princeton. He writes a book explaining Quantum theory from the kind of Copenhagen Bohrian point of view, and he's in Princeton, Einstein's over at the Institute for Advanced Study. And he's kind of saying, "Gosh, I would like to get my nerve to go ask Einstein what he thinks of my book." Right and he's actually trying to get other people to approach Einstein [laughter] And as this is going on, he gets a message from Einstein and Einstein says, I read your book. Would you like to come talk about it? And the story is he goes to talk to Einstein.

0:39:09.8 SC: It's like 1950, '51 something like that?

0:39:11.5 TM: Yeah, '51, I think. Einstein has read the book and he, Einstein says, I think this is the best, the clearest exposition of the Copenhagen approach that I've ever seen. I mean, he was really trying hard, but Einstein raised some objections, [laughter] And the story is, and I think it was Gilman who tells this story, it was Bohm's roommate at the time that, that Bohm goes, meets Einstein comes back and he reports when he comes back, "I'm back to square one"

[laughter]

0:39:40.4 TM: He was convinced by Einstein's objections. He just, he couldn't answer Einstein's objections. He realized that he really didn't understand what he himself had written. And within a year he discovers this other thing, which was the same thing that de Broglie had done, and apparently independently, as far as I can tell. I mean, I was told recently, I haven't done the research myself, but again, it's not hard to find that theory if you're looking in a certain direction.

0:40:03.6 SC: Right, right.

0:40:05.3 TM: And then he publishes these papers, this two part paper in 1952 a theory that introduces so-called hidden variables into quantum mechanics and gives you all the right predictions. And this was something that people had believed had been proven mathematically impossible by John Von Neumann when Von Neumann wrote his book.

0:40:26.7 SC: Yeah.

0:40:27.8 TM: So this is of course, a shock.

[laughter]

0:40:27.8 TM: It's like you're not supposed to be able to do that. And then you look at the paper and you say, wait it does look like you can do that. And we now understand that Von Neumann had made not a mathematical mistake, but a kind of conceptual mistake, which was caught at the time by Grete Hermann.

0:40:45.1 SC: Grete Hermann, yeah.

0:40:46.3 TM: And people pointed it out, but you know who's listening to Grete Hermann.

0:40:50.5 SC: Right. When, when Bohr and Heisenberg are saying.

0:40:52.6 TM: Yeah.

0:40:53.1 SC: The opposite. Yeah.

0:40:53.3 TM: So in '52, the, the theory sort of gets reborn. It's under Bohm's name. And Bohm worked on expanding it to cover spin and some other, I mean, you know, it's a kind of very basic theory at beginning. And then you start adding bells and whistles to cover more and more phenomena.

0:41:09.2 SC: And it might still have disappeared, but John Bell became a fan of it right?

0:41:15.7 TM: Well, Bell, it's not that Bell started out as a fan of it, Bell was a fan of this. He was a fan of saying, "I understand what this theory is claiming." There's no mystery about it. There are these particles, there's this wave function. Here's what the wave function does. Here's what the particles do okay? This is a picture of the world, like it or lump it. And when I first learned about this theory, and I was very skeptical about it from Shelly Goldstein, Shelly helped me a lot at one point in our discussions, 'cause I was being very obstreperous. And he said, "Look, surely you'll admit this is a theory of something."

[laughter]

0:41:53.6 TM: Okay. It says, there're these particles, there's this other thing. Here's what they do. Now, just in your mind, imagine a world as this theory describes, wouldn't it be interesting if that world looked very much like the world we live in [laughter], which turns out to be true.

[laughter]

0:42:09.7 SC: This is always good advice when people have trouble thinking about new theories because they always wanna fit it into their own boxes, right?

0:42:16.6 TM: Yeah.

0:42:17.2 SC: And if you just ask what it would be like if it were true. You'd make progress.

0:42:19.7 TM: Right, right.

0:42:20.9 SC: And it solved the... It addressed the problem that Einstein had raised about locality by saying, yeah.

0:42:27.3 TM: Yes. So this is what I was gonna say. So Bell and Bell tells a wonderful story, right? Bell says, when he first learned quantum mechanics, he was very puzzled by it. He didn't understand what was going on. And he thought, you need to add something to it. You need these additional variables, the wave function's not the whole story. And he said he was told by his physics professors, no, no, no, no. Von Neumann has proven it can't be done mathematically, proven you can't add any additional variables and keep the same predictions.

0:43:00.0 TM: And Bell says, at the time, he didn't know any German and the book had not been translated. And so that's where it sat. He just didn't know why. He didn't understand. He couldn't read Von Neumann's proof, but he was told there was a proof. And then Bell says he just gets up in 1952 and reads this paper that's been published and says, "Wait, Bohm has just done what everybody's telling me Von Neumann proved couldn't be done. Something's gone wrong here." He quickly convinces himself that the mistake in that case was on Von Neumann's side, but he sees that the theory has this manifest nonlocality. And he asks himself, surely what Einstein must have been asking himself, can you get rid of it? Can I have a kind of theory like this, but without the nonlocality in it that still makes the right predictions? The quantum mechanical predictions. That's the very precise thought that Bell had. And when he finally got some time off on sabbatical to sit down.

0:44:04.5 SC: 'cause he couldn't do it during his regular work anymore.

0:44:07.7 TM: Ah, no [laughter] But it's, but the amazing thing is, of course, when you see Bell's proof, it's a few lines of algebra. It's not like, you know, proving Fermat's Last Theorem. Okay. It's not hard mathematically. When I first read about this stuff, which was in, in the Scientific American in an article by Bernard d'Espagnat back in '79, I guess, it's not looking back on it, it's not the greatest article in the world, but enough was there that just with a little math, you can see wait, [laughter] wait, I see the problem, right?

0:44:40.0 SC: Yeah.

0:44:40.1 TM: I really, this just stands out at you. So what happens with Bell is Bell what Bell realizes, "Wait a minute. No, you can't get rid of the nonlocality out of this theory. You can't get rid of the nonlocality out of any theory." I mean, throw away quantum mechanics and start from scratch from entirely different principles. Bell's proof is not about quantum mechanics, it's just about certain experiments and the correlations that you see in the outcomes. You can't have a theory that is local in a well-defined sense that won't predict that. Quantum mechanics does predict it. And more importantly than quantum mechanics predicting it, the world does it. Right. That's why the Nobel Prize went to, went to Clauser and Aspect and Zeilinger recently for doing the experiments that showed that the violation of Bell's inequality actually occurs.

0:45:32.9 SC: And as you and I both complained about in separate occasions, the Nobel Committee blew it in their press release.

0:45:38.8 TM: Yes, they did. [laughter] They said that what they had done was what proved that Von Neumann was right. And you can't have hidden variables. Which is just the irony there is so delicious. [laughter] Because Bell became, you know, Bell then became the, probably the strongest advocate of this Bohm's theory or de Broglie's theory, which is a theory with additional variables. Why? Because he said, well, what's the objection? You can't object as Einstein did. I don't like it 'cause it's non-local, because now I understand, okay, the nonlocality is just in the world. Right? Suck it up. [laughter] And it's a clear theory and so on. I mean, at one point Bell says he's going through all these different ways of approaching quantum mechanics and he says, well, the pilot wave approach shows the best craftsmanship. But then he says, but is that he says then wonderfully, I mean he always made these cutting little remarks. Is that, is that a virtue in our time?

[laughter]

0:46:34.4 TM: Right? Do people appreciate good craftsmanship anymore or do they like just things that sound woohoo, you know, amazing and blow your mind 'cause it, at some level, it's a much more pedestrian kind of theory, you say, yeah, there's some particles they move around. Yeah. There's this wave function. That's the kind of funny thing, but I'll explain how it, you know, how it plays into the architecture here.

0:46:54.2 SC: It was a rough time for the philosophy of quantum mechanics.

0:46:58.3 TM: Yeah. [laughter]

0:46:58.4 SC: In certain ways, but, okay. So the, I think that there are any physicists out there who are not into the foundations of quantum mechanics and they're listening, they're very confused because one uses the word locality in different senses sometimes. I mean, if you ask a quantum field theorist is, is their field theory local? They would say yes, it's very, very local. So what exactly do you mean by this word?

0:47:20.4 TM: Good. So, and, and there's even a technical mathematical condition that's imposed in quantum field theory that they will call locality, has to do with commuting operators. Okay, so what's going on? Well, one thing to say is that there is, there's Bell proved there has to be some kind of influence or effect if Alice and Bob, if I set up a pair of particles in a special state called an entangled state, and anybody who's heard about this, at least have heard the word entangled but send the particles off way in different directions. So they're, you know, hundreds of light years apart, one into Alice's lab and one into Bob's lab. The intuition of causal locality basically says nothing that Alice does can have any actual influence or effect on what Bob sees. Right?

0:48:14.9 SC: Yep.

0:48:15.5 TM: That doesn't mean there can't be correlations between what they see. I mean, everybody knows people give this example, you tear a dollar bill in half and send it to sides off in different directions. Of course, Alice seeing one half immediately informs you that Bob will see the other half. But that's not Alice doing anything that affects Bob at all. That's just trivial.

0:48:34.0 SC: It was always true that [0:48:34.4] ____.

0:48:35.8 TM: Everybody understood that. Everybody understands there could be correlations like that, but those correlations are already explained by what happened at the source. And so nothing non-local is going on in that case. Bell showed, no, you can't get the kinds of correlations you see in quantum mechanics by that kind of local story. There has to be a sense in which what Alice does actually makes a difference to what Bob sees or the other way around. That's causal nonlocality. Now the next question that arises is, "Oh, could you implement that in some way to allow Alice to send a message to Bob?"

0:49:14.6 SC: Right.

0:49:15.5 TM: Signal Bob, right. Tell Bob whether they're gonna meet for, you know, lunch today or not. And the really interesting, I mean, a very interesting thing is that at least under certain pretty clear idealizations, you can kind of look at the way quantum mechanics work and say, well no, actually, even though you can't get rid of this connection, you also can't use it to signal. And, and the person who proved that first was also Bell. People like to call it the No-Bell telephone theorem, so he was perfectly aware of it. Right? You can't use the nonlocality to signal.

0:49:48.3 SC: I'm sorry that's a feature in Copenhagen, Everett, Bohm. Whatever you want, you never use.

0:49:56.2 TM: Yeah, well, okay, now this is, now I'm gonna, now I'm gonna tread off into treacherous territory.

0:50:02.2 SC: Good. That's why we're here. It's late in the podcast.

0:50:02.3 TM: Okay. If you say, and I think it's just a bad thing to say that the fundamental postulate of say relativity has to do with signaling that you can't signal faster than light. And you say, oh, well then this is still a local theory, but that just, I think first of all, signaling isn't even the right kind of concept that should be in the foundations of any theory, just as measurement shouldn't and just as observation shouldn't. And you know, Bell proved there is this kind of non-local causation and maybe I can't use it to signal, but so what? But then the next thought is, gosh, if there really is this real physical non-local connection, somehow, why can't I use it to signal.

0:50:46.2 SC: Yeah. [laughter]

0:50:46.7 TM: I mean, you'd sort of offhand think there must be some clever way to do it. Now I'm gonna report something to ask that question. Of course, you really need a well-defined theory on the table in front of you, right? Because you have to ask, well, what can I do with this physics? Quantum mechanics tends to use these things called Hermitian operators or as they call them observables. And they sort of the mathematics of that is, is what they look into to ask certain questions. But if you have an actual theory in front of you, you're not restricted in that way. You can just say, well, can I do something? Can I twiddle this and see something there? It looks like in the Bohmian theory with spin, there's reasons to believe you could signal faster than light.

0:51:35.1 SC: Oh. Oh, this could be most popular podcast ever.

0:51:38.5 TM: You weren't expecting that, were you?

0:51:41.4 SC: I was not.

0:51:41.5 TM: It is the... Personally. I suspected there should be a way. Siddhant Das, who's a young researcher, has been looking at something which amazingly people had not looked carefully of at, which is in this pilot wave theory, when you add spin, okay? That's an additional physical degree of freedom. People usually just don't fiddle with it. They kind of deal with spinless particles, you know, at this level to understand things. When you add spin and when you look at, there's a very specific thing you can look at that standard quantum mechanics doesn't have an obvious way to even treat. All right. This is the key to understanding this, that is arrival times. So if I...

0:52:31.5 SC: This is a well known issue.

0:52:34.5 TM: Yes, it is a well known issue, although people just kind of put it out of their mind. So if I have an electron now, so we're confined in a little box, which we kind of know how to do and at some moment I open the box and then I've got a screen. And at some time a flash appears on the screen, one thing you can ask is, well, what was the, what was the transit time? How long, what was the time gap between when I opened the box and when the flash occurred? And it turns out, if you ask quantum mechanically, and in quantum mechanics, you of course have to do experiments many, many times and you get kind of statistical results. You get a distribution of results. It's nobody agrees how even to make that prediction using standard quantum mechanics because there is no Hermitian operator that corresponds to arrival times. Okay?

0:53:19.5 SC: Mm-hmm.

0:53:19.6 TM: In the, in this pilot wave picture because you have these particles and they just move. It's kind of easy at least to ask the question, according to the theory, how long would it take a particle here to get there [laughter], right? So you can actually do these calculations and when you do them with spin, you start to see that there's a spin dependence of the arrival times which you really weren't expecting, but it shows up. And if that's right, it's already a question of whether you'll see that this is something you could check in the lab. We're trying to find people who will do it [laughter], but you need specialized equipment and you need the motivation to do it. If there is that spin dependence of the arrival times, then there's a very quick argument. Oh, you could signal.

0:54:03.4 TM: Now the signaling would be very subtle. I mean, it's not like you could flip a switch and a light would go on there, but it would essentially say if Alice orients a magnet in her lab in a certain way, Bob will suddenly start seeing a slightly different distribution of arrival times than if it's a oriented another way. And that would be any kind of information, right? It doesn't have to be a clean a 100% strong signal. Any signal. Right. Any Shannon information.

0:54:34.6 SC: Sure.

0:54:35.4 TM: And this suggests that, yeah, you actually should be able to do that.

0:54:38.8 SC: Okay.

0:54:39.6 TM: Now there are lots of, still, there's lots of arguments within the community about whether this is right. How far do you have to carry the analysis into the observing equipment? I mean, lots of stuff. Maybe in a couple years people will say no, this argument breaks down for some reasons we don't quite understand yet. So I don't want to say this is nailed down.

0:54:56.6 SC: No, that's okay.

0:54:57.4 TM: But it's there.

0:54:57.8 SC: Maybe cutting edge. Yeah.

0:54:58.9 TM: And it's a really hot interesting, interesting topic. And one that could be tested in a lab. Some of it you could do today.

0:55:05.5 SC: I'm not completely surprised 'cause both you and I have written papers about energy conservation and its failure in quantum mechanics, which anyway anyone could have done in the 1950s.

0:55:13.4 TM: Yeah.

0:55:13.5 SC: I just didn't ask the question. But is this a fact that you're talking about now? Would it be a difference between Bohmian mechanics and other formulations of quantum? Or is suggested you can't ask the question?

0:55:25.9 TM: Yeah, it's a little hard to say because, and again, I'm now just reporting what Siddhant tells me, and he's done all the research. He says, if you go into the standard physics literature about arrival times, you'll find 20 different suggestions about how to do it. They tend to agree with each other in the far field. That is, if there's a long time between when you release it and when you detect it, all of these different approaches converge on the same answers. But in the midfield there are places where they will not converge. So what can you say? I mean there's, it, it's not clear what predictions the "standard approach" even makes.

0:56:02.3 SC: Got it.

0:56:02.7 TM: So you could say if the, if the pilot wave approach makes a clean prediction, it's clearly a different theory because look, it's doing something precise where the arrival doesn't even have anything precise to say.

0:56:13.7 SC: The other obvious question is you've been simultaneously talking about whether you can send a signal faster than the speed of light, but also working in the context of non-relativistic quantum mechanics.

0:56:23.2 TM: Yes.

0:56:23.4 SC: Where there is no speed of light so...

0:56:25.4 TM: That's right.

0:56:26.5 SC: Do we have to talk about relativistic quantum field theory to have this conversation?

0:56:31.7 TM: Well, we certainly don't have to talk about it to have the conversation we've had up until now. 'cause even before relativity, if you said, look, Alice is in a lab, a hundred billion, billion million miles away from Bob [laughter]

0:56:42.2 SC: Yeah.

0:56:43.2 TM: Can Alice do anything in her lab that would make a difference to what shows up in Bob's lab?

0:56:48.0 SC: That's a [0:56:49.6] ____ question.

[overlapping conversation]

0:56:49.7 TM: Your natural thought would be no. I mean, you know, [laughter], even if there were some effect, even if there were a fast effect, it would drop off with the square of the distance. It would tail off. There's, you know... So you can have the discussion about non locality, even in a non relativistic. Now it becomes sharper in a relativistic context because then you mean faster than light where there is a light cone structure. There's kind of an objective thing you mean there that's very sharp. So you can sharpen it up in the relativistic context, but I think you can still reasonably have it, it's not like all of your discussion of the non-relativistic case just falls to the floor when you notice this relativity.

0:57:30.1 SC: My halfway-informed feeling is that Bohmian mechanics looks quite natural in the non-relativistic regime, and it's a little bit more of a challenge to see how we should just do quantum field theory in that picture.

0:57:45.9 TM: Yeah, there is no doubt. They are both conceptual and technical challenges when you go from standard quantum mechanics where... And one of the signals here for people who don't know, is that in quantum mechanics, you talk about a system having n particles and that's it. Unless you add Summer takes them away, it's gonna be n particles forever.

0:58:10.8 SC: But that's not the world.

0:58:12.5 TM: That's not the world. We now know that there is some phenomenon that we call particle creation, annihilation, the particle numbers can change, and so you need to count for that. Whether, I mean, there are lots of subtleties here, one subtlety is the thing we call particle of creation and annihilation is it really... And what I mean by that is when Dirac, and he's just a standard guy who was talking about this, he didn't have particles really come into existence, He said, Well, there are always infinitely many particles filling up negative energy states, and what we call the creation of a new electron and a new anti-electron, a new positron is really just lifting an electron that was there all along in a negative energy state up into a positive energy state and leaving a hole behind, and so it looks like there's a new electron and the hole is the thing we call the positron, now, that's a story that would account for the phenomenon without really creating any new particles, it's just moving them around in energy space as it were, so there's certainly... You have to account for the phenomenon, you have to account for the phenomenon.

0:59:18.6 TM: Yeah. I can take a really high energy photon and gee, there just appears now an electron and a positron or some things that satisfy certain conservation laws. We need to account for that phenomenon, whether it really deeply requires new particles or do you wanna give up the particle? The other thing to say is the pilot wave picture is not tied to particles, it just says there's something that the wave function is guiding, it could be particles, it could be a more field-like thing. There's a kind of way of doing Bohmian field theory where you replace the particles with fields, but the same kind of architecture, and the other point to make, which at least needs to be made, is that people can say, Gosh, but there's no good Bohmian quantum field theory. Standard quantum field theory has been plagued by mathematical problems and run away singularities numbers you try to calculate and they all come out infinity, it's not as if it's all conceptual lightness and clarity in the so-called standard picture, so you don't wanna hold this picture to a higher standard of mathematical precision and so on, then you're allowing for the theory that you happen to use.

1:00:35.3 SC: Okay, that's perfectly fair, but I'm just trying to get straight for the listeners out there, if I'm a particle physicist and I just calculated the rate of Higgs bosons decaying to two photons and something like that. Is that stuff I can do just as well in the current Bohmian framework? Or is that work in progress?

1:00:54.3 TM: Okay, so one of the things that people have looked at is, suppose I remain... And again, there are different moves you could make, you could say, I'm gonna give up the particles all together and put something else, more fuel-like in, or you can say, No, I'm gonna stick with the particles, but I am gonna allow literally, the number to change. I'm gonna allow particle creation and annihilation. You can do that. That, how you would work that out in a kind of pilot wave picture was done. Has been done for 30 years ago, I guess. And what you do is you say, in the original picture, you have a fixed number of particles and all they do is move around and change their configuration, but now I say no, no, the real space of possible states is, there's a zero particle state and a one particle state, and then two particle states and then three, so there's this whole thing called fock space, which is familiar to any of the physicists you're talking about, and you say, No, that's what I'm now gonna say. The thing is always in a particular location with a particular number of particles, sometimes it just moves around in one of these sectors and the number doesn't change, and sometimes you can jump up or jump down, which corresponds to particle creation annihilation.

1:02:06.2 TM: I now need dynamics for that, it turns out that dynamics tends, instead of being deterministic, to be indeterministic, the easiest way to do it, you say at any given time, there's a certain chance you'll jump up and create a new particle or jump down and lose particles. You can do that, and what falls out of doing that is the mathematics of standard quantum field theory.

1:02:28.5 SC: Okay. Just so I don't lose it. Among people who are fans of Bohmian mechanics right now, there's a camp that thinks maybe the hidden variables are field-like and there's another camp that thinks maybe there's collection of particles that can change that number.

1:02:45.7 TM: Yes. There is still particle [1:02:47.1] ____.

[overlapping conversation]

1:02:47.6 SC: So we're no sure.

1:02:48.6 TM: That's right.

1:02:48.7 SC: There's it's not a lead.

1:02:48.8 TM: And there's a lot of things that are debated within it, so here's another question, people, when you start out, you're always treating electrons and gluons as particles, but... And they're all fermions, right? They're all half integer spins.

1:03:01.4 SC: But the gluons are not.

1:03:03.7 TM: Oh, sorry. Sorry, I meant quarks. Then people ask, Well, what about the photon? Is that a particle? Are there photon trajectories? And it turns out you can try and develop a theory in which there are and a theory in which there aren't. And people work on both sides and there's no agreement in the community, and you've asked, Well, what would be the advantages of doing it this way, what would be the advantage of doing it this way? So it's a general architecture of how to develop a theory that can be implemented, of course, in many different ways, and so even within the Big House, right, there's the rooms in the big mansion of people who are working in different directions.

1:03:46.7 SC: With regard to the idea of sort of stepping back from the fields and going particles as your ontology, two things come to mind; one of which is against that idea, one of which is for it. So which one do you want me to tell you first, to then you an respond.

[laughter]

1:04:01.7 SC: The worry I have is...

1:04:04.4 TM: Yeah. Give me the bad news first.

1:04:07.9 SC: Something like The Higgs mechanism, where it seems that the Higgs boson lurking as a field with an expectation value throughout all of space is playing a crucially important role in explaining phenomenologically observable properties. Is that something you can...

1:04:20.2 TM: I think I just answered that, which is, it sounds like you're the kind of person who would say, I'd rather... Yeah, I'll treat my fermions as particles and I'll treat my bosons as field essentially, which is...

1:04:32.4 SC: Oh, okay.

1:04:34.1 TM: Yeah. I won't introduce particles associated with the energy spin objects. That I think would make you happy. I don't have to say there's a Higgs particle to work it into the theory, and there is a reason to think that you're gonna work bosons and fermions and can enter into the theory in different roles.

1:04:52.7 SC: Okay, this is gonna undo the good news that I had, so I don't know if you know this, 'cause I only recently learned it, but Richard Feynman, when he was inventing Feynman diagrams, part of his motivation was to get rid of quantum field theory and replace it with another theory, particles again, and we now think of Feynman diagrams is a tool for understanding quantum field theory, so he changed his mind, but his motivation, I'm told, was the cosmological constant problem.

1:05:19.0 TM: Really?

1:05:19.4 SC: Yeah.

1:05:20.7 TM: No, I did not know that.

1:05:21.3 SC: Yeah, I was told it, I need to find the reference for it, but the idea is if you have fields, they all have these zero point energies and they add to infinity or you cut it off and it gives you a big vacuum energy, but if you have particles you don't have that problem. So if I were a Bohmian particle ontology guy, I'd be claiming a solution of the cosmological constant problem. That'd be my advice.

1:05:42.6 TM: That's a connection I was unaware of. My understanding, and again, this is not based in anything other than what I somehow picked up on the street, was that one way anyway... Let me put it this way, how Feynman thought about it, I'm not sure. One way to think about Feynman diagrams, which runs contrary to the way people often talk about them, is that they were just a mnemonic device, right? I have a mathematical equation that has an infinite number of terms that I need to sum up, and of course the issue gets bad when that some goes to infinity. But anyway, I have this... All these terms I need to add up. And that the diagrams were just easy ways to kind of remember what the different terms looked like it.

1:06:28.4 SC: To figure out what [1:06:29.5] ____ had to do.

1:06:30.0 TM: Exactly. And now, the way I say people don't do that is they often pointed those things as if they are literal pictures of stuff going on, "Oh, this is doing this, and this is doing this, and all the stuff is going on, and it's a booming buzzing confusion of virtual particles that are running around." That talk, I don't think makes any sense on any view.

1:06:49.2 SC: I'm on your side. Yeah.

1:06:50.8 TM: So if it's just a mnemonic technique, then you're really... It's as if I like to say, suppose you wanted to calculate the volume of a sphere and you just love cubes, and you say, "Okay, well, I'm gonna, first of all, figure out the biggest cube I can sit inside a sphere and calculate its volume." And now I've got these other little six round pieces sticking out, now I'm gonna find the biggest cube I can get out of each of those and then add those and then more cubes and more cues and more cubes. And you can see, Okay, there's gonna be an infinite sequence of these volumes that you now need to add up to get the total volume of the sphere, but you say, but you shouldn't think that the sphere is this really complicated thing, it's actually quite simple, and in fact, here's another way to do this calculation that is straightforward, and you don't do an infinite sum and gee, it's just 4/3 πr3.

1:07:40.1 SC: No, I think that's actually an important point because lots of modern physics is motivated beyond the standard model particle physics searches for our next best theory is motivated by questions of naturalness and fine tuning and so forth, and I think... I don't know what the right way to think about those problems is, but I think that not only do people talk as if there really are virtual particles popping out of existence, which there aren't, but they also think that somehow nature starts with a classical theory and then adds quantum corrections on top of it, and nature doesn't do that, and that might change our perspective here.

1:08:17.1 TM: Right. And I think there's a... I think you're exactly right, and I think there's a methodological point here, which is that because we start out learning Classical Physics and it feels familiar to us and we're familiar with the math of it, there's this great temptation if someone says, "Oh, the way to come up with a good quantum theory is to start with a classical theory and then do this thing we call quantising it," we put the hats on, we turn certain variables into operators and well, most of the time, it's clear how to do that. Sometimes it's not quite clear how to do that. But you think, "But this doesn't make any sense." The world... Yes, the world behaves pretty well the way classical theory predicts at a certain scale, but that's because something emerging out of a very non-classical physics, and there's no reason to think that the right way into that physics is to start with the approximation, and then twiddle with it, it's gotta go the other way around, you need the fundamental physics and then hopefully you can understand why the classical approximations work as well as they do, but I think people are tempted because you have this kind of kind of algorithmic thing of, "Oh, just take a variable and put a hat on it and turn it into an operator," but conceptually, that's just not the right way to think about it.

1:09:36.1 SC: The reality doesn't work that way.

1:09:37.8 TM: Yeah. Reality doesn't work that way.

1:09:38.0 SC: Yeah, that's right. So good. [1:09:39.2] ____ Like that we can find all of our points of agreement against the consensus despite the fact that we have some disagreements, but this question of the speed of light, I'm gonna go back to the experiment, the possible signalling because I do not know about that work, I don't know whether it's right.

1:09:54.0 TM: Yes, it's right.

1:09:56.6 SC: So that's okay. We're putting it right there, but the idea of looking hard for possible experiments to do, I think is a crucially important one. My impression is that your typical Bohmian on the street thinks there is no experiment to do to test Bohm versus conventional quantum mechanics, whatever that is, do you have a feeling about that?

1:10:17.2 TM: I do, and this is a discussion I am in right now, arising out of this work. That is a lot of people in the Bohmian camp will kinda say, "Oh, but we know that it won't make different predictions and "standard quantum mechanics". Now, I think that's just wrong. I think there are kind of [1:10:38.5] ____ toy cases that make it obviously wrong, but leave those aside, I just try to understand what the argument was and I can't get it. And I think some of this is defensive because there's this... And you stop back a second, and you see, "This is so unfair," standard quantum mechanics or whatever it is. Anyway, you can make lots of good predictions with it, then somebody comes up with a new theory and then they say, "Oh, does your theory make any new predictions? If not, why should I pay any attention to it?" Now, the first thing you notice is, But wait, if these two theories had been discovered in the opposite orders...

1:11:14.7 SC: Is an asymmetry.

1:11:16.4 TM: You would be going the other way around. So this can't be from a logical point of view, a good objection that why should I take your theory seriously, 'cause it makes all the same predictions as my theory because that doesn't make any sense.

[laughter]

1:11:30.7 TM: But I think that is an objection that physicists often made, and so there were certain kind of defensive procedures that were put up to say, "Oh no, we're not saying their new predictions," and then you'd have to go on. My view is a sharp theory, again, standard Quantum Mechanics has certain vagueness in it; if you have to talk about measurement, if you have to talk about observation. Because those are vague terms, these are the objections that we had. And so you expect a sharp theory will certainly ought to differ in some ways, and you should be looking hard at where those differences might be.

1:12:14.1 SC: So unfair question, what fraction of the Bohmians out there agree with you about that? [laughter] Is there a vibrant little...

1:12:19.3 TM: It's not an unfair question, but it's a sociological question, which I just can't answer. I don't know, I don't know there are... And even the... As it were, the Bohmian camp is divided into sub-camps and probably the statistics and the different ones are different, and probably... My guess is weirdly enough, it probably correlates with age.

1:12:41.5 SC: Oh, that's not weird at all.

1:12:43.5 TM: I think younger people are more open to certain things than people who've been through a lot of battles, but I really don't know.

1:12:50.9 SC: Where are these Bohmians of which you speak, are they in philosophy departments or physics department, or?

1:12:55.2 TM: Yeah. A lot of them. So there were a lot of students in Europe, and I'm sorry, this is gonna be just a little hard for me, Detlef Durr, who's at the LMU in Munich, had always a dozen graduate students. Unbelievable what he did that nobody else I know has ever done to promote young people and bring them into this.

1:13:25.8 SC: And he was in a physics department?

1:13:29.6 TM: He was in the physics... He was in the math...

1:13:30.3 SC: Oh, math maybe.

1:13:34.5 TM: Physics/Math department. And he was just the most wonderful, caring doctor, father and looked after these students, he could not get many of them jobs in academia, a lot of them have gone into other industry, whatever. They did their work, but there are a lot of young people who came through that program who know it. Who know the theory and work on it. Siddhant Das, who I mentioned was one of Detlef's last students. And some of his students, a couple of them are in [1:14:06.7] ____ Tübingen, so there are more of them in Europe who've come through LMU and through his influence. Then the other thing that's happened is a lot of physicists who realized they're interested in foundations, notice that if you have credentials as a philosopher, you can literally more easily have an academic career in a Philosophy Department as a philosopher of physics than you can in the physics department as a physicist.

1:14:35.4 SC: Still true, yeah.

1:14:35.5 TM: Also Detlef's student some of them have switched over. Also, Detlef student, Dustin Lazarovici, who is now in the Technion in Israel, there are people who are around, and it's more than there used to be, the total number is up. What the effect of losing Detlef 'cause Detlef died during COVID, I don't know. I mean, nobody right now is doing what he was able to do, and we have to hope that some dedicated person with the right psychology and so on can kind of take his place and help people, or.

1:15:13.6 SC: Do you think you started by talking about why you don't need to make the sales pitch to philosophers. They get it.

1:15:20.6 TM: Yeah.

1:15:22.0 SC: How deeply do they get it? How much has quantum mechanics changed what philosophers think about metaphysics?

1:15:31.2 TM: So this is an interesting question. In philosophy, I would say there are three camps of people who do what goes under the name metaphysics, I just gave this talk at Rutgers to the undergraduates about metaphysics in science or metaphysics in science, enemies or friends or what?

1:15:49.6 SC: All the same thing?

1:15:49.9 TM: Yeah, yeah, exactly. That was my conclusion at the end. So you jumped right to the end.

1:15:56.4 SC: Sorry.

1:15:56.5 TM: But no, there's an interesting history, 'cause really, there's a certain point where they're clearly considered to be enemies, I mean it can happen anywhere. I would say that metaphysicians, people would self-identify as metaphysicians in philosophy now fall into kind of three camps. There's a camp that's sometimes is called analytic metaphysics, which tends to be doing things which don't seem to care much about science, they're asking questions that don't... It's not obvious that any scientific theory would bear one way or the other on the question they're asked.

1:16:33.3 SC: Could you give us an example?

1:16:37.2 TM: These are now just gonna be words, other universals or tropes? Okay, so this is the kind of thing that a philosopher of a certain kind will understand and they'll be fighting with each other forever about universal and tropes. And it's very hard if you say, "Well, did Newton believe in universal or tropes? You'd say, "Well, Newton didn't care."

1:16:53.0 TM: It's just not an issue that is built into the guts of that theory in any obvious way. Then there are... Then there's another camp that goes by the name Scientific Metaphysics for the reason of saying that, what I think is obviously correct, metaphysics is about describing what exists at the most general scales. And if you're interested in what exists in the physical world, you better pay attention to science. Not to say that the scientists can just answer those questions, but they do have relevant things to say, to offer. But then that group kind of subdivides into two. The ones who then specialize, say, to do philosophy of physics. Now that really requires learning some math and some physics and at a reasonably good scale. Some people who have full PhDs in physics. But you certainly have to devote some time to just studying math and physics to do this in a kind of specialized way. And then there's another group of people who've clearly acknowledged the relevance of the physics, but they're not experts. And so they like to listen to the experts and try and figure what they can draw out of it, right? Without themselves delving so deeply into it. I would say that's how the landscape goes.

1:18:05.8 SC: But do they talk to each other these people? Or they do go to the same conferences?

1:18:12.9 TM: Yeah, there are certainly... They do talk. Well they're in each other's presence. There are a lot of specialized conferences...

1:18:22.4 SC: Words happen.

1:18:22.8 TM: But there are conferences. I mean, I was just invited to be on a panel at the Metaphysic Society of America. And I'll tell you, the people in that society are not people you'll ever run into. They were doing a very different kind of metaphysic, but they were happy. I talked about what I do and the way I think about it. And they didn't throw bricks at me. It wasn't the way they thought about it. They were interested.

1:18:45.4 SC: They were interested is important there.

1:18:47.5 TM: Yeah. They were interested and they appreciated having the conversation. And I mean, philosophers tend to be pretty open to talking about weird stuff because that's more or less comes with the territory, whatever strikes them as weird, you still don't just reject it.

1:19:03.3 SC: I guess what I'm getting at, and I'm not trying to presume the right answer cause I honestly don't know, but to what extent can we imagine smoothing over the conceptual differences between thinking about physics and thinking about metaphysics? It really should be thinking about reality is what we're doing, right?

1:19:20.2 TM: Yeah, I do need to mention here that, again, you can be interested in metaphysics and not be a physicalist. So you'd say, "Part of what I do really is not gonna be informed by physics." If I'm a mathematical platonist and I'm worried about mathematical reality. And I say, "That's just not part of physical reality. So I don't care how your physics comes out."

1:19:42.4 SC: Fair enough.

1:19:44.3 TM: That's a fair thing. So not all of metaphysics has to be hostage or all that interested. It depends on where you're focusing your attention.

1:19:51.3 SC: Yeah, sure. Okay, we actually... Barry Loewer was another person we had on recently and Barry, he said something not on the podcast sadly, but in informal conversation that was even stronger than something I would say. And I think he was being humorous about it. But he said he remembers very vividly being struck the day he realized that most problems of philosophy could be cured or solved by statistical mechanics.

[laughter]

1:20:21.6 SC: And I think that he has a different view about you than I did. So we're running out of time a little bit, but I wanted you to have a chance to give a little bit of your view of the arrow of time and things like that, which is the other giant thing that people who care about foundations of physics spend their time worrying about.

1:20:38.4 TM: Right, so let me actually start with the last thing first and just get it out in the open so people ought to at least be aware of it. I would say it is kind of, everybody has this idea there's space and it's kind of three-dimensional, at least at large scales. And if I talked about, say the north, south direction, at a certain point, if I went on to say, by the way, space goes north to south, it doesn't go south to north. You'd look at me funny, and you'd say, "What in the world are you talking about? The spatial thing, it just... There are two directions and they're oppositely directed, but space itself doesn't go more one way than the other. I don't even know what you mean." On the other hand, the person in the street you say time goes from past to future, doesn't go from future to past. They'll again nod at you and say, "Yeah, tell me something new." Now I happen to agree with that, that as I think time is fundamentally different from space, temporal structure is fundamentally different from spatial structure and that temporal structure does have a fundamental directionality to it. And spatial structure doesn't. This turns out to be a very contentious view among philosophers and physicists for funny reasons, I would say. I don't know that we have the time to go into, but it surely is the minority view.

1:21:57.3 SC: And it's a little bit... Just to clarify, it's a little bit different than there are physicists out there and philosophers who will question whether or not time exists or whether it's fundamental. But here we're talking about the direction of time, not time.

1:22:12.5 TM: The direction of time, yes. Yes, that's right. It would be even harder to deny the existence of time altogether. But certainly the directionality is hotly contested. I think time itself has a direction. Now, it is clearly the case that there are temporal manifest, temporal asymmetries on any view. If you give me two pictures of somebody 10 years apart, usually I can look at them and figure, "Oh, this was the younger, and this is the older," right? Because people tend to wrinkle up and... Now, that's not... Maybe they had cosmetic surgery. I could get that wrong. But there are certainly lots of very reliable temporal asymmetries. And those need an explanation. Simply saying, time has a direction would not explain, all by itself, any of those observable, temporal asymmetries or what people sometimes called arrows of time. So I think all of that is a perfectly good object of study. Even if you believe, as I do, that one of the things that you can invoke is that time is itself direct. That doesn't give me these observable asymmetries like that. But it might be part of the explanation.

1:23:26.4 TM: How does it help? Since people like David Albert and Barry and myself think that you don't need an intrinsic arrow if you just have a initial condition that is doing all the work.

1:23:38.2 SC: Well, because a lot of the explanation... We can do this in different levels, but let me do the first level. What do I mean by explaining something? Well, often I think it is a kind of causal explanation. I talk about cause and effect, and now you say, "Okay, but what distinguishes cause and effect?" Offhand in many cases causes proceed the effects. I mean, if people often talk about running a movie backward or imagining that, which of course you can literally run a movie backward, but then the end sequence comes before the title sequence. You still have a direction of time. If you took that as seriously as a movie of something in the normal direction, also all the causal structure would be reversed. What were causes would become effects and therefore the structure of causal explanation would be different. And so you have to get a bit deep and statistical mechanics does come into this here in a way that we couldn't get on the table clearly in a short period of time. Where you not only have causal explanations where you say this particular precise physical state gives rise to this later one. But other kinds of statistical explanations where you say this kind of physical state, not, I haven't precisely, but I'm gonna constrain it in certain ways...

1:24:57.2 SC: There's a class of various things.

1:24:58.3 TM: Gives rise to this kind of state. This is the sort of thing that you take a box of gas and you say, if you wait a while, it'll eventually relax to an equilibrium. And that of course shows a directionality to it because from this initial state, you can say in five minutes it'll be the equilibrium. If you look at the equilibrium state at a generic level, a kind of fuzzed out level, you couldn't say, "Oh, five minutes ago it was in this non-equilibrium state." So there's kind of asymmetry there. And that's connected to certain asymmetries in statistical explanation, probabilistic explanation. Very interesting and deep hard topic.

1:25:38.6 TM: A bit different than worrying about say relativity, which we might say is about space-time structure. Quantum mechanics is about material structure. Statistical explanation is about a kind of explanatory project and what goes into it and what succeeds as giving statistical explanations of things. But I do think that reversing the direction of time, you might have a very good, what you consider to be a very good statistical explanation of something as seen as it were in one direction of time. And if you look at the other direction, you say, "Wait, there's no explanation at all." The explanatory part went away. All I can say is a massive coincidence, or else to be teleological and say, the future is somehow affecting the present. But I take it to be one of the great discoveries of the scientific revolution to get rid of that kind of teleology.

1:26:31.4 SC: Does your view that time has an intrinsic direction affect what we would call micro-physics, the standard model of particle physics, for example? Or could it?

1:26:40.8 TM: Sure, it could. I think that it plays into your fundamental picture of space, time, structure. And of course, one needs an account of space-time structure to even begin to write down what you think of as laws of physics. And I think there are very specific ways in which having a direction, well certainly having a direction. Well, let me just give you a very concrete thing, one could say, suppose I have a kind of grid, like a street grid, and I have two points on it and you ask me, "Okay, how many continuous paths are there that'll take me from A to B?" And typically the answer, if the grid goes on forever, it's infinitely many. 'Cause it can kinda go short way or around, or far around, go way out and come back. Now, suppose I put some arrows on that grid, so they're one-way streets. Now it's not at all clear that this number will always be infinite. Now it might be quite tightly constrained. The directionality gives me a resource there that a lack of directionality doesn't have. And I think that's actually quite important and plays out in trying to understand the laws of physics. But it's much more complicated.

1:27:53.8 SC: It's much more complicated, but that's what you're thinking about these days, in fact.

1:27:54.4 TM: That's one of the things I'm thinking about very hard. I mean, I've been working on thinking about what space and time could be like if they were actually not continuous, but discreet at fundamental scale. I worked a long time and did six chapters just on space, and now I have to bring in time. And the time part really has to have a direction just for it to work it has to have a direction. And you have to understand how the directionality comes in, sort of thing.

1:28:17.5 SC: Sorry, when you say for it to work...

1:28:19.9 TM: For this scheme I have...

1:28:22.1 SC: For your scheme. Okay.

1:28:23.9 TM: To be a kind of thing where you could do a plausible looking sort of physics with.

1:28:26.2 SC: Do you have a catchy name for your scheme?

1:28:28.6 TM: Yeah, full discrete geometry.

1:28:30.8 SC: Full discrete geometry. Okay, good. And the time directed-ness has to be part of the story too.

1:28:35.5 TM: Yeah, it's fundamental in it.

1:28:36.8 SC: Okay. All right, good. Well, we're looking forward to that. Will it be a book at some point?

1:28:39.2 TM: So am I.

1:28:42.1 SC: You said chapters, so...

1:28:42.5 TM: I got really stuck. Yeah, well I'm up on chapter eight now. The spatial part went real well and then it was like, "Okay, part two, let's bring in time." And then you realize it's a tricky business. There are a lot to think...

1:28:53.5 SC: Is it quantum talking, or classical?

1:28:55.8 TM: This is all classical geometry. But quantum field theory... There is a connection to quantum field theory, which I'll just state briefly. We talked about problems just in doing quantum field theory, just mathematical problems. One way that this kind of Bohmian or pilot- wave approach for quantum field theory was done, it was done by Bell. It's called Bell-type quantum field theory. And one thing he does is he puts it on a lattice, that is, he doesn't do it in continuous space, he does it on a kind of discreet lattice. And you have these jumps from lattice points to lattice points. That turns out to be very important to have a structure where you can do the math in a clean way.

1:29:33.4 SC: Okay, looking forward to the book coming out. We'll pre-advertise it here. So here's the last, I'm not even gonna call it a question, thing to respond to. When we talk about quantum mechanics, and as we say, you had to give up something maybe that was hopeful or intuitive or whatever. And even in relativity it seems like maybe the world is a little bit different. Is there hope for fundamentally understanding why the world is this way? In retrospect, can you say, "Oh yes, it had to be quantum mechanics." Or are we just stuck with it?

1:30:03.8 TM: Yeah, I think people like to say that, why the quantum? You'll get this thing. I think, "Oh, come on. Just grow up." No, I mean there's gonna be foundations. There's gonna be... And one of the really hard questions in this whole topic is the question, where do I stop digging? Where is my spade turned? Where do I say, "Okay, I've hit a foundation here. It looks like this is a plausible place to stop." And all you can say is, "Yeah, this is the way it is. Could it have been some other way? Yeah. Could have been some other way. It isn't, this looks like a plausible place to stop." If you're never gonna be satisfied, okay, then you're never gonna be satisfied. And problem is there are dangers in both directions. If you're too easily satisfied, you'll stop digging when in fact, you could discover a lot of interesting structure lying underneath where you've stopped. But if you're too pigheaded, you're gonna get to the bottom and still be banging your head against it forever because there is nothing underneath it. So there's a kind of connoisseur's feel for where's a reasonable place to stop. But I don't think you're gonna be stopped by some master principle that says, it could only have been this way. How could you expect that? The world is kind of contingent?

1:31:23.8 SC: Well, that seems like a reasonable place to stop. So Tim Maudlin, thanks very much for being on the Mindscape Podcast.

1:31:28.9 TM: Can I actually...

1:31:29.1 SC: Of course.

1:31:29.6 TM: Before you do that, because I'm here and I'm... Have many voices, many ears than I ever kind of normally will have. And I'm going to just take advantage of you...

1:31:39.4 SC: Please.

1:31:42.2 TM: And say, one of the things I'm doing is trying to promote foundations of physics and the way that I'm doing it is I've founded this thing called the John Bell Institute for the foundations of physics. And we are at the very moment, a kind of critical moment where we're trying to buy a physical place for us to live. And if anybody out there likes people who talk about this stuff and thinks they should have a place to go where they can meet and talk to each other, we have a GoFundMe or you could go to www.johnbellinstitute.org if anybody would like to help us out in any way. It's our moment of need and we would appreciate it greatly.

1:32:20.2 SC: Good to be reminded of that. I will also mention that in the intro and put links on the webpage.

1:32:25.5 TM: Thank you.

1:32:25.9 SC: And I can mention too, because it's an audio podcast, Tim is wearing a very fetching John Bell Institute polo shirt right now. So that could be swag, I don't know. Is that a GoFundMe?

1:32:35.8 TM: To get the shirt you gotta come to the place.

1:32:39.2 SC: Oh, they come to the place to get the shirts. Good.

1:32:40.0 TM: The only exceptions of that are my parents who are 95. And I said, "You can have shirts without actually going." But this, you can only wear the shirt if you've been there and you've seen this beautiful island in Croatia and enjoyed it, and then you get a shirt.

1:32:54.3 SC: But it is actually an important thing. I'm glad you mentioned it because it's a reminder that ideas are great and conversations about them are great, but institutions also matter for getting these ideas out there.

1:33:06.8 TM: And this is the problem, is this field is an academic orphan. Foundations of physics doesn't fit. It's kind of doesn't fit in physics departments and it doesn't really fit in philosophy departments.

1:33:18.4 SC: Tell me.

1:33:19.2 TM: And so if it's gonna live, a place has to be made for it.

1:33:22.9 SC: All right, let's look it up on the internet. So in that case, Tim Maudlin, thanks very much for being on Mindscape podcast.

1:33:28.0 TM: Thank you.

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