Theories of Everything with Curt Jaimungal - Emily Adlam & David Wallace: The Quantum Interpretation That Divides Physicists

Episode Date: July 20, 2026

SPONSORS: - Go to https://www.plaud.ai/curt and use the promo code "CURT" to get a Plaud device today - I personally subscribe to The Economist. TOE listeners get 35% off the annual subscription. No other podcast has this! https://economist.com/TOE This is a podcast all about parallel universes, and whether we have any right to believe in them. David Wallace, professor of philosophy of science at the University of Pittsburgh and a leading defender of the Many Worlds Interpretation, joins Professor Emily Adlam, his former student, for a rare joint conversation. The central fault line: Adlam argues no one living inside a many-worlds universe could ever rationally confirm the theory describing it, since Everettian probability may undermine the evidence used to support quantum mechanics itself. Wallace disagrees, but takes the challenge seriously. From there they range across the nature of the wave function, relational quantum mechanics, QBism, and scientific intersubjectivity — two sharp, collegial minds disagreeing productively about what kind of universe we actually live in. TIMESTAMPS: - 00:00:00 - Wave Function Ontology - 00:05:35 - Everettian Probability Problem - 00:11:45 - Relational Quantum Mechanics Limits - 00:19:15 - Quantum Gravity Frameworks - 00:24:50 - Subsystem Decomposition Approximations - 00:31:10 - Testing Wavefunction Collapse - 00:38:35 - Scientific Intersubjectivity Crisis - 00:47:55 - Many Worlds Rationality Constraints - 00:58:50 - QBism and Physical Reality - 01:05:05 - Conservative Philosophy of Physics LINKS MENTIONED: - Emily Adlam's Papers: https://scholar.google.com/citations?user=RlTqjZMAAAAJ - David Wallace's Papers: https://scholar.google.com/citations?user=Z35CeHAAAAAJ - Pointer Basis Of Quantum Apparatus [Paper]: https://libertadacademica.com/Cuantica/ModernaHist/Zurek%20b.pdf - Information Is Physical [Paper]: https://arxiv.org/abs/2203.13342 - Relative Information, Relative Facts [Paper]: https://arxiv.org/abs/2510.11349 - What Kind Of Relationality Does Quantum Mechanics Exhibit? [Paper]: https://arxiv.org/abs/2502.06991 - Models Of Wave-Function Collapse, Underlying Theories, And Experimental Tests [Paper]: https://arxiv.org/abs/1204.4325 - On Gravity's Role In Quantum State Reduction [Paper]: https://link.springer.com/article/10.1007/BF02105068 - Real Patterns In Physics And Beyond [Paper]: https://philsci-archive.pitt.edu/23888/ - Learning To Represent [Paper]: https://philsci-archive.pitt.edu/23224/1/learning_to_represent.pdf - Operational Theories As Structural Realism [Paper]: https://arxiv.org/abs/2201.09316 - Relational Quantum Mechanics [Paper]: https://arxiv.org/abs/quant-ph/9609002 - Measurement Outcomes And Probability In Everettian Quantum Mechanics [Paper]: https://www.sciencedirect.com/science/article/abs/pii/S1355219806000694 - Quantum Probability From Subjective Likelihood [Paper]: https://arxiv.org/abs/quant-ph/0312157 More links (past guest episodes) at https://curtjaimungal.substack.com FOLLOW: - Spotify: https://open.spotify.com/show/4gL14b92xAErofYQA7bU4e - Substack: https://curtjaimungal.substack.com/subscribe - Twitter: https://twitter.com/TOEwithCurt - Discord Invite: https://discord.com/invite/kBcnfNVwqs - Crypto: https://nowpayments.io/donation/TOE - PayPal: https://www.paypal.com/donate?hosted_button_id=XUBHNMFXUX5S4 Guests do not pay to appear. #science Learn more about your ad choices. Visit megaphone.fm/adchoices

You disagree about whether we're living in one of the vast number of palli index. It doesn't make sense to believe in interpretation of quantum mechanics, which says we shouldn't believe quantum mechanics. This is David Wallace, Professor of Philosophy of Science at the University of Pittsburgh and one of the leading philosophical defenders of the many world's interpretation. Years prior, David supervised Emily's undergraduate thesis at Oxford. I think it's much more likely we'll learn something about epistemology from resolving that, but learn something about physics. Professor Emily Adlam, who's also been on the podcast.

before, argued that no one inside many worlds could ever confirm many worlds. Both believe the philosophy of physics is just as important as the physics itself. I don't know which interpretation is right. On this channel, I. Kirchai Mungle interview researchers regarding their theories of reality with rigor and technical depth. Today, I bring you Emily Adlam and David Wallace's first joint interview starting with one of the deceptively simplest questions in quantum mechanics. What is the way functioned?

We'll start with you, David. it's the representational tool in quantum mechanics that encodes the physical features systems have. So there's a take on the way function that just says it's epistemic, but the take I want to say is it's not a thing in itself, that's a category error, but it encodes the properties things have and the relations they stand in. And for a given system, the wave function of that system just encodes all the properties. that system has. Emily, do you broadly agree? To some extent. I mean, I think there are two types of roles that states sort of tends to play in physics, and one is to represent the sort of occurrence properties of the system at a time, and another is to encode predictions about how the system's going to behave in the future,

and at least in classical physics, we typically expect those things to kind of come together, that the predictions are just sort of given by the current properties at a given time. I think there are reasons to think that in quantum mechanics that might not be the case, and so we might want to make more of a distinction between the properties that a system currently has and the prediction we want to make about its future. And so for that reason, I'm slightly skeptical about whether we should assume that the wave function represents properties or whether it just rather represents a sort of state of predictions. People ask a question like, what is the wave function ontologically?

Does it exist? Is there such a thing as a universal wave function? Is that all that there is? I know there are some views that all that there is is this universal wave function evolving forward, but there are two different uses of the word universal wave function. So there's a bunch of things mixed together there, and I should say incidentally, just to clarify when I'm coming from, I'm answering these questions flat out from the Everettian perspective, I think is probably right. If I was teaching a general philosophy of quantum mechanics class,

I'd be a lot more ecumenical about what I think the wave function is. I'm a bit more dogmatic as to what I think the right interpretation is than Emily, so I'm probably willing to be a bit more committal there. So I would say that to ask what the wave function is ontologically for everyone is kind of the wrong question to ask. It's not a thing, or at least to suppose it's a thing, is a big substantive extra step. There's nothing in even very realist approach to quantum mechanics to say that the wave function itself is a thing. It's an if a representational reading of its right, then it encodes properties things have. And to go to Emily's comment, if some more influential reading of its right, then it's a result. If some more influential reading of its right, then it's, encode something else. But the point is it's not, you know, it's a tool we use in our theories either to represent or to predict or to infer or whatever. So then the question is the universal wave function is can we talk about the universe as a whole as a quantum system with properties that can be represented? And then I think the answer is probably yes in the sense that we

do so fairly effectively when we're doing quantum cosmology. It's not what we normally do, but we can do it if we want to. Yeah. I mean, I think The question is there a universal way function is a good question, partly because I'm interested in relational approaches to quantum mechanics. And one of the main commitments of the relational approaches is that there is not a universal wave function. There could not be a universal way function because wave functions always have to be attributed relative to something else.

So that, I guess, is one of the central points setting apart various relational approaches from the effort interpretation, for example. And I think one reason to be a bit skeptical about the use of a universal way function is because the source. sort of central observation that the relational approaches want to make is that in practice, we do always use wave functions relationally,

with those sort of using them relative to some experimental setup or to some particular reference frame or what have you. And so all the evidence we really have for way functions is of the sort of relational use of them. So there's somewhat of an extrapolation to step beyond that and say wave functions can be used in this kind of absolute way to describe the universe as a whole. So that's one reason to have some skepticism there. Emily, would you plant your flag down on the relational quantum mechanical interpretation in the same way that David plants the flag down on the Everettian side?

No, I mean, I don't know which interpretation is right. I also think that the relational approach, as it is currently formulated, can't be right. I think it has a bunch of problems that need fixing. I do think the sort of central core idea that there's something relational about quantum mechanics is promising, but I think more work needs to be done to develop what that should look like. Now, clearly, David is no fool by any stretch, but you're not buying, you're not jumping where that flag, has dominion. So why? Why are you not changing, you're getting a passport there? Why do you only visit? Do I even visit? Do I even visit? I had visited in the past, yes. No, I could definitely see

the appeal of the Evers approach, not at least because I think it's the only approach which sort of reliably works at all applications that we know of, and so that's obviously a very appealing feature. And the worries that I have about Eversa approach, not really. The worries that I have about Everett had to do with the role of probabilities and empirical confirmations. Obviously, there's sort of long-debting discussion about how to make sense of the probabilities in Everett, and there are various strategies you can sort of use to address that. I'm not presently convinced that any of those existing strategies give you probabilities that are robust enough to play the kind of role that I think probabilities need to play in confirmation to make sense of quantum mechanics. So for that reason,

I am doubtful and would like to see a sort of a clear demonstration one way or the other that this can be done. So is it the case that Emily, you're not arguing that Everett is false. It's that Everett, under some interpretation, I know there are different slices, there's a Wallachian view, and there's a Saunders view. I just spoke to Saunders, and they're not precisely the same. So under some interpretation that it can't be rationally justified from inside the Everettian universe, which is different than just saying Everett, that interpretation is false. What are you precisely saying, Emily? Yeah, well, the worry is that if there is no adequate solution to the Evereating probability problem, then if you believe that you are in an Everettian world, it would follow that you wouldn't be able to use relative frequencies and so on as empirical evidence for a theory and the way that we actually have used them to support quantum mechanics. So it then looks like you shouldn't think that quantum mechanics is right, but it doesn't make sense to believe in interpretation of quantum mechanics, which says we shouldn't believe quantum mechanics. So the whole thing looks a bit self- undermining if the probability problem can't be resolved. So it comes to. down to, can it be resolved? Yeah, I mean, in a certain sense, I agree with that, but I'd put the logic a little differently. I mean, here's the thing about Everettian quantum mechanics. The framework of unitary quantum mechanics, so the mathematical core of what Everett's using, which I'd say is the standard equations of quantum mechanics, but also commitment to the fact measurement is a physical process,

commitment to the fact there's no epistemic role of what's going on, there's no special role of observation. The difference between the quantum and the classical is just a matter of dynamics and scales, the physical systems can be studied in isolation. That kind of framework, I'd want to say, is robustly scientifically successful. There's no internal problem inside physics practice in doing that. And that's different, I'd say, from way away in quantum mechanics in, say, prior to the 1980s, prior to a lot of development in decoherence and measurement theory. And at a meta level, I'm just much more skeptical of the premises of any philosophical argument that says we have to reject that framework than I am of the result there.

I mean, I think we're, we don't have a lot of precedent of situations in which we've got a completely successful scientific theory where we have some entirely external to it, philosophical, conceptual constraint. I think if there is some kind of clash between deeply held principles of epistemology and the framework of quantum mechanics, I think it's much more likely we'll learn something about epistemology from resolving that than learn something about physics. So, I mean, I'm entirely happy to get into the weeds of how I think we should think about the details of the probability problem inside Everett. But that's my kind of starting point. I think it's an important philosophical

problem. I think we can learn a lot about probability from thinking about it, and dialectically, since a bunch of people are ever sympathetic but worried about probability, insofar as there are things that can be said to assuage those worries, it's worth saying. But I would need a hell of an argument to convince me that on sort of our priori philosophical grounds, we should reject Everett on the grounds it clashes with some epistemological

principle we have, however compelling that principle might seem to be. Well, Emily, you can give that hell of an argument if you like. and just imagine that you all are at a conference and I'm not here, and you can also feel free to ask questions to one another. Yeah, well, I mean, I should say that I also am not keen to reject quantum mechanics or any large part of the framework of it. You know, obviously, my ideal interpretation would be an interpretation that preserved quantum mechanics as is and yet was not the ever interpretation.

And, you know, obviously I don't have a complete version of such interpretation to offer, so that is a problem. I mean, because I think Everett has this very significant problem and because I nonetheless think that quantum mechanics, as it currently exists, is so well demonstrated, that gives me some faith that there must be another interpretation, which would work. But yeah, I think I agree that currently don't have such a thing

and so we should be looking for it. So interpretation of that formalism, so you wouldn't think something like an Adrian Kent's proposals for photon-based final state theories would, would be as attractive? Just trying to don't get the parameters. I think because the way in which quantum mechanics is put together is quite sensitive and quite difficult to alter successfully,

ideal interpretation should either change nothing or change as little as possible. So for that reason, I'm quite skeptical about sort of boom-style interpretations or collapsed-style interpretations, which require changing a lot. I'm hopeful that there might be ways to have other types of interpretations, which are not Everett, but which nonetheless maintain very significant aspects of the existing formalism. That's one of the reasons why I interested in the relational approaches, because as I understand, one of their goals is to try to provide an alternative, which nonetheless preserves most of the formalism of quantum mechanics.

Yeah. So it might be worth distinguishing between what you might call Everett Light. Exactly, let me back off. Sorry, I've tangled my chain of thought. Let me actually just put, since I paused, let me just save for Kurt. But I'm playing, like, we're doing the Let's Talk to the Conference framework, but Emily and I have talked about this stuff a lot. So the way we actually want to talk about is inevitably going to have to have a fair amount of pedicology. If we just have a direct conversation about this, I think it's going to get. Oh, that's totally fine. Have a direct conversation. Don't feel like you have to then, and by the way, audience, what I mean is so and so. Who cares about that? Get into the weeds. Oh, right, fine. So maybe distinguish between modification light and pure interpretation.

So there might be modicatory moves that, as you say, are less kind of thoroughgoing than Dynamical collapse theories or inval variables. I'm really skeptical, but if they were, then, yeah, I can see that's a window. You might be had to explore. Yeah. And then there's kind of genuinely pure interpretation. So if there's a pure interpretation of QM that's not ever hurt, then, yeah, I'm happy to say that's very much on the table and we can talk about it.

I mean, from that point of view, if we think about the relational idea, I guess the question is, and I think this has come up in some of your work, there's a sort of tension where if you, it's hard to find a happy medium that doesn't can meet you either to a relatively thoroughgoing modification of formalism or basically claps back to Everett. I mean, I'm not, if you take the sort of original Revelli idea that the core content of relationism was just. simply the statement that we can't state a quantum state other than in the context of some other system. I mean, I'm not even sure that differs from ever at a level that isn't a relatively kind of high metaphysics disagreement. I'm not even sure I couldn't be persuaded that was an equivalent way of talking. I'm not, I mean, you're right that the genuine use of a universal wave function. I don't think there were zero cases we used that in physics, but there are exception of views. So I'm pretty relaxed if one lost that. But I think for that very token,

if one had that light a form of relationism, then your sort of adamant versions about Everett are going to come be just as bad there. And then the worry is, is there a relationism that is going to avoid the kind of concerns you've got that doesn't end up committed to a sort of far-going modification that, as you say, is going to run into a ton of technical problems? Yeah, no, I agree with that. And there's been a discussion recently in the literature over relational quantum mechanics about how to define quantum events and whether this can be done in a precise way within the formalism of quantum mechanics. Having thought about this for a while, it seems to be obvious that you can't.

This can't be done. The only thing you can really do is what, as a recent paper by Ravelli and DiBiaggio, just basically proposing a version of Everett saying that you can look from the outside and you can see something that kind of looks like in measurement as the things become entangled and discuss this with them and said, isn't it's just Everett? And the conclusion is, well, yes, essentially, it has some different philosophical emphasis but it is essentially the same thing and it's going to have the same problems. So, yeah, I think the conclusion of this discussion for me is that there is no relational approach

that's significantly distinct from Everett, which preserves the entire formalism of quantum mechanics and adds nothing to it. I'm still hopeful that there are directions you could investigate, which would be relatively minor modificatory strategies and which might would still leave quantum mechanics intact in the domains where it's successful. That's obviously more a promissory note than a, a, well-ve-ed-a-d-a-d-a-d-deal-ad at the stage. Yeah, I mean, that's fair enough. And it's worth saying, I mean, look, I'm perfectly pro-people working on things like that. I'm perfectly pro-people

working on dynamical class and then they'll do it as a, I mean, as a, if we're doing physics, rather than philosophy, then absolutely let's explore the theory space. But then I'm separately interested in the question, well, okay, given the actual best theory we have at the moment, what are we going to do with it? How do we think about it? I mean, I mean, just to just to clarify what you have in mind for the sort of a really move. My guess is we're doing something like the following. I can be a relationist, but I can still say, okay, fine, you go and do the Shredding of cat experiment and I retreat to Alphson-Torai because I'm distressed by violence to cats. While I'm on Alpha Set tori and you do the experiment and you're outside my light cone,

then even the relationist, if they're not going to be too sparse in the attributions of states, is going to say something like, well, your state relative to mine is something like a superposition of a life cat and a dead cat. Yeah. Yeah. I mean, the question is being posed in the literature is about, you know, how do you say when and how a measurement has taken place in ACU. And the answer they want to give is that you look from the outside and you see that adequate correlations have come about between these two systems. And then once this is a sort of smooth process, which happens as a matter of degree. And then that's all you can say about it. Which seems to me more or less exactly what you would also say within a sort of Everett style story. So, yeah, yeah. There's no, there's no room in that story for a specific individual. the judicial outcome to Everica. Well, I mean, after all, the Revellian relative state is perfectly definable inside the Everett interpretation.

Yes, right. And as you point out, the universal state is not doing a hell of a lot of work in that story, but the state relative to me may be enormously indefinite, which gets you the problems, yeah. So I'm hearing an awful lot of, I agree, I agree, I agree. But where's the disagreement? Well, I think the disagreement is that I, you know, despite seeing the attractions of the Everett picture, I still think the epistemic issues are significant enough to make me rule it out. And so

I therefore think there must be some other answer, which I think there are various directions to look in. I think there are interesting new ideas coming in from more recent work in physics and quantum gravity and so on, and so I'm hopeful that there are other options that we just haven't explored yet. But yeah, I mean, I agree that kind of of the existing interpretations that we have, the Everett is the one that, apart from these epistemic concerns, you know, works best and is most easily applicable. So just to push your priors on this, how committed I use these epistemic things. Suppose we prove some frankly fairly implausible nago theorem that says that no minor tweak of the formalism of quantum mechanics is going to get you away from the ever

interpretation. You have to do a thorough going modification of the formalism along the lines of a dynamical collapse theory in variable theory. That's the only way you could get around it. Are you still confident enough of those epistemic arguments you want to say, nonetheless, what we conclude from that is that it must be that there exists some third-game modification of this kind? Yes, I would conclude that, yeah. I would be disappointed because that it looks like very hard work, but I think the epistemic concerns are for me serious enough that, you know, unless someone comes up with that, what an argument that I would find more convincing to show that the erratian probabilities can play the right kind of role, then just as a

matter of kind of internal logic, I just kind of flipped it. So, I mean, let me push a little bit. I mean, yes, obviously as a matter of internal logic, if you have confidity premises, then you have to reject some. But I mean, you know, I love philosophy. Some of my best friends are philosophers. I could get paid by my way to answer to philosophy. But does analytic philosophy really have the kind of track record to give one that level of confidence in epistemological priors? I mean, look, if I thought quantum mechanics itself was the end of the story of physics, and there was no more physics to be done, and the only remaining obstacle was the epistemic

concerns that might be a different situation, part of the reason I'm willing to investigate at the possibilities is because quantum mechanics is clearly not the end of the story, and there are things we don't understand about physics. And so there are certainly routes to explore which might modify quantum mechanics anyway and which might potentially also give interpretations, which I find more coherent. Good, okay, that's reason enough. So, sure. So, yes, if you're inclined to think someone like quantum gravity is again going to modify the core quantum formalism, then, sure, there's a natural synergy.

I can certainly see that. I mean, and conversely, I mean, I think string theory is very substantially our best strategy for quantum gravity at the moment. I think, and that's just a unitary, I modify quantum theory, modulates some slight delicacies. about the problem of time, so is quantum gravity says asthmotic safety. These are all, those are what I would count as the series run-up processes. So I'm hardly moved, but the fact all of those are quantum theories. Yes, although I mean, I think I have some concerns about all of those having been formulated in ways which rely on a theory that we haven't interpreted properly yet.

And so I wonder if there's some interaction between ongoing difficulties and formulating theories of quantum gravity and difficulties in interpretation of the existing theory. Sure. Yeah, I mean, you can go out of the way. Again, I'm probably a lot more optimistic about how well we're doing this strength of it, than you are. But, no, sure. I mean, I think it's, I mean, look, as I said before, I have a lot more time for sure, let's change the formalism as part of a sort of ongoing project in physics. I'm quite unmoved by it as a pro rome rate. For me, the hardest part about most conversations is actually what happens after. That is the follow-ups that I'm supposed to send, or the ideas that I have on walks later that just escape my mind by the time that I sit down.

Fortunately, I've been using Plod, P-L-A-U-D, and the shift is super simple. I stopped trying to hold everything in my head. Plod starts with hardware, which is what makes it different. There's the Notepin S, which is a wearable, and it's completely hands-free. I use it when I'm moving around, when I'm out for a walk, when I'm thinking out loud, even when I'm in the shower, wherever I don't want to pull out my phone. And when I need to capture phone calls or more intentional conversations, I use the Note Pro. It magnetically attaches to my phone, records calls and meetings, and keeps everything organized afterward. What Plod truly does is it

turns what I've heard into something that I can actually use, summaries, action items, follow-ups, without the mental overhead of reconstructing everything later. It basically becomes a searchable record of your own work. So if your work lives in, conversations, whether that's research, medicine, law, or just a huge amount of meetings, it's worth looking into. It's also built around enterprise security standards, S-L-C-2, H-I-P-A-A, and G-D-P-R-compliant. Check it out at plod.a-I-I-S-U-R-T, and use code C-U-R-T for a 15-% discount. This video is sponsored by Plod. All opinions are my own. What do you mean, Emily, when you say that some of the approaches to quantum gravity inherit some interpretational issues of quantum theory?

Well, I mean, some of the approaches to quantum gravity have difficulties or have sort of conceptual issues they're trying to overcome with respect to the role of observers and the way observers tie into the theory. So, I mean, a problem of time can be thought of that way. The problem of time is something to do with understanding. the nature of our experience of time and how we end up kind of looking into the world despite the fact that it seems as though there's no sort of evolution in some versions of quantum mechanics. Another issue is that quite a lot of,

in Luke quantum gravity, for example, the most straightforward way to do a calculation is kind of like fix an external classical space-time boundary and then do quantum gravity sort of inside that boundary. And so you still seem to need some kind of like Heisenberg cut in much the same ways as you do in sort of standard non-relatistic quantum mechanics. So there seem to be some conceptual issues and difficulties within quantum gravity surrounding the role of observers.

And since that was sort of already a problem, as I see it, in standard quantum mechanics, it seemed to be at least plausible there's some link between the problems we're having in quantum gravity and the earlier problems in interpreting quantum mechanics. I'm not sure how crucial it is, but let me suggest those are problems in quantum cosmology, not quantum gravity per se. I mean, if we want to study an isolated subsystem, we always individuate subsystems, spatia temporally. and that's in classical GR likewise, we'll individuate our subsystems and locate them against the sort of classically specified background, we'll put the ban positions there. I think we have fairly satisfactory quantum theories of gravity, at least in string theory. I agree we have no satisfactory quantum theory

cosmology, and part of the reason for that, I guess, is precisely this system localisation. I mean, I'm slightly sceptical about making a division between those two problems in a sharp way precisely because subsystem decompositions themselves are one of the things that, start to look quite odd in the point of view of certain kinds of canonical quantization, for example. So, you know, while I agree that you can look at an individual subsystem and derive useful predictions out of that quantum gravitation league, the separation between those two problems itself seem to me to be a kind of approximation and something that we shouldn't

sort of rely on conceptually. I mean, I could get in the weeds of quantum gravity. Let me really strongly object to the... idea that because something's at approximation, we shouldn't rely on it conceptually. I mean, almost everything to the approximation. Match the selection isn't an approximation. I still want to use it to explain adaptive complexity. Sure.

No, that's there. Now, we can use the approximation if our goal is to understand how to make predictions for the quantum gravity or some effectively isolated subsystem. I think nonetheless, if you're trying to understand, I guess, what quantum gravity is and what it's telling us about spacetime and how it links to earlier frameworks and how our experience and a quantum non-rowensive quantum mechanics merges out of it. I'm slightly skeptical that you can tell that story in the complete and satisfactory way while ignoring these more cosmological issues because they all, they seem quite tightly tied together in the framework of quantum gravity in a way

that they may be a not in earlier physics. How do you think that goes? I mean, that's not obvious to me. If I take an isolated system, I put it in the box. I idealise the box in certain respects. I do quantum gravity inside the box. And I look at the degree in some regimes, I get a big black hole and I get an interest in quantum mechanical staff on the surface of black hole. In other regimes, I'll get perturbations of flat space time and I can just do QFT. I mean, that's the regime in which we recover low energy quantum gravity and one field theory from. well, as it were, string theory is the place where that's most concretely successful.

I mean, I wouldn't faintly want to suggest that's the end of the story. I'm just not sure why the cosmological versus non-cosmological and strong gravity versus non-strong gravity bits don't commute. Well, I mean, for example, from a relational point of view again, one of the sort of recent directions of investigation in the relational formalism has been looking at the way in which subsystems decompositions depend on a choice. reference frame. So given a fixed subsystem decomposition, you can for sure do a description of it and

describe the physics of just that subsystem, sort of relative to the choice of background. But it seemed to me that if you want to tell a complete story about how all that is emerging from what it is underneath, then you are going to have to take into account the fact that the subsystem decomposition itself is not fixed, and that's, and that the procedure for choosing it is part of the story. Okay. Yeah, I mean, I could believe that might depend a bit on how much the relationism you're using. And as first I'm just concerned that we do, I mean, in some ways,

I'm reiterated my sort of general complacency move here, that we do seem to have models of isolated systems and we do actually seem to make quite a lot of progress in getting out low energy from high energy, even while recognizing that the cosmological situation is still much for opaque. Yeah, that's fair enough. Yeah. And I think I agree that we can do a lot of things usefully, but there are also certain questions that seem to be important about the way these things are tied together. Yeah, that's fair. Okay, so to close the chapter on the Everettian side, the reason that you don't buy into it, it has to do with some epistemological problems, which David is also willing to concede,

but doesn't see the problem as being so strong that David would abandon, whereas Emily, you wait that a bit more strongly than David does, and David is saying that, look, are you going to suggest that there's some large modification to quantum theory that wouldn't imply Everett? and then Emily's saying, well, maybe because we still don't have some final physics, maybe there's something about the interaction between gravity and quantum mechanics that isn't so straightforward. Yeah, as I say, I'm hopeful that there's a minor modification which will succeed, and we don't have to make a really large modification.

But if push comes to shove and a large modification is the only thing that will work, then I'm willing to do that. I'm willing for someone else to do the work. I think I would be inclined to think it has to be done in that case. So the place where I slightly disagree, Kurt, is it's not that I think these epistemological problems are slightly less problematic than Emily thinks they are. I think these epistemological problems are solvable in a completely satisfactory way with no residue. I'm entirely happy with their solution inside Everett. Arguing for that was the main theme of the earlier phase of my intellectual life.

what I was saying in this context is that even persuade me that what I see as satisfactory philosophical solutions here don't work. I am much more inclined to think, well, we need to think harder about the philosophical issues of epistemology and probability so that we can work out how we can solve them inside the formalism than to think we should abandon that. formalism. And to some extent, I mean, at that level, it becomes inevitably inconclusive. I mean, there's a matter of one sort of priors as to what's likely to break, which is kind of what we're talking about. There's also a matter of division of labour. I mean, my, and I think this is a

disagreement between me and Emily, my feeling is, even if it were true that we need to make large and thoroughgoing modifications of the formalism of quantum mechanics, this is not something within the skill set of the typical, even highly technically minded philosopher of physics. So in terms of what do philosophers have to contribute to the measurement problem, it's more likely to be in the space of exploring somewhat more philosophical moves, either Emily's kind of minor modification, quasi-relational explorations, or the more sort of flatly let's explore Everett. But I don't want to give the impression that I think these are profoundly difficult problems that we don't know how to solve, but we can expect to be able to solve. I think they're solved just fine, thanks.

Okay, I'm curious then if you all disagree for the same reason. So do you all dislike, say, dynamical approaches or a Penrosean approach or a timodlian approach to interpretations of quantum mechanics for the same reasons, or do you all not buy into them for dissimilar reasons? So why don't we pick the most fun one. Perhaps it's more interested to talk about what's different between these two ways of doing things. There is an enormous difference, in my view, between approaches which lead to at least quasi-tractable tests of the return to quantum mechanics and theories that are intended to be modifications of the formalism of quantum mechanics in ways that are undetectable.

So if you think about dynamical collapse theories where, you know, the collapse, you know, the collapse of the wave function, Schroening his cat, resolving into a live or dead, but not both, is something that's really supposed to happen as a matter of microdynamics. It's premature,

and I think not really the point, to look at those theories and say, well, what would the world be like if they're fundamentally true? You want to think of those theories as ways of parameterizing the space of what collapse might look like.

If one of these theories turns out to be right experimentally, that's a crack opened into new physics that would probably look different from anything we can imagine. The point of those theories in the forms we have now is to let us do experiments or work out what experiments we could do and try to test those theories. I mean, I think collapse theories are good science, and one of the ways to tell their good science is that I think they're within 10 or 20 years for being completely ruled out experiment. What about a Jonathan Oppenheim approach to quantum gravity, where gravity remains classical, and somehow the coupling between them is stochastic?

Does that have any bearing to Everett? Well, only if it's true, Everett's false. If on Oppenheimer's approach, then effectively you have a wave function collapse. So if I can have a superposition of the matter, the idea of the Oppenheim approach, and it has a long pedigree, is that the gravitational force isn't sourced by the matter distribution, it's sourced by the sort of smeared matter distribution of all the terms in the quantum superpositions. If the quantum superpositions evolve unitarily and don't collapse, that will rapidly and radically violate experiment. So those theories are only viable in the context of some kind of collapse mechanism, which is normally tied to gravity itself.

So the idea is if I have a sufficiently spread out mass distribution in a superposition, then the dynamics will somehow have to select one of those. And so the gravitational field can still carry on being sourced by the smeared mass distribution, because there's not really that smear that smeared anymore. So obviously if that's right, then these macroscopic superpositions, whichever it's committed to, don't exist. Yeah, I mean, I think that those styles of approaches which maintain sort of a classicality of space-time and gravity are appealing because there are a bunch of both conceptual and technical difficulties that come from trying to make sense of suppositions of space-time. So I can see the appeal of avoiding them. I think the technical challenges of making such a thing work are strong and would probably have low credence that it will work in the end but I think because it has such nice features,

there's a good reason to explore it. I mean, I'd say I'm, I think the technical problems of having superpositions of geometry are massively overstated. I think the technical problems of not having them are much more severe. I think we have a mathematically fairly satisfactory theory of quantum gravity and energies below the plank scale, well below the blank scale.

There is some, you know, it has a whole bunch of interpretational questions, some of which are just the measurement problem, but bracket that. So I'll, so, I mean, I hesitate to disagree with Sir Roger Penrose, but I do what I, you know, it's worth saying penrose is very much as a minority view here. However, again, in a certain sense,

it sort of doesn't matter. We're probably within 10 to 20 years of just being able to test this theory empirically. I have money riding on the fact that it will fail the test and be eliminated. I would be very happy to lose the bet because it would open an amazing vista of new physics. We'd learn a ton of stuff. I'm not expecting to learn those things, but the point is, will, the experiments to test these things are in the kind of prototyping and applying for grants level. I'm not enough of an experimentalist to know the exact timing, but it's probably isn't going to be 10 years, but it's not going to be 50.

Is it a polymarket bet? No, no, it's just a friend of mine who believes in Claps theories, which one of us has to buy the other one. I forget a bottle of good wine, I think. David, you have some recent work about realism. So what is realism firstly? Sorry, let me adjust to the change. Realism very loosely is the picture that our scientific theories

are telling us things about what the world, world is like, what, or perhaps more mundanely what the systems they describe is like, not just how they appear to be at the scales of which human senses can engage with them, but at the scales beyond those senses. So, classically speaking, the anti-realist doesn't believe in electrons or nuclei of atoms. They don't think we have reason to think those things exist, or maybe they think even talk of those things is just a fancy way of coding our. predictions for experiments. The realist thinks, and again there's lots of gradations of this, but loosely speaking, yes, there are electrons, yes, there are atoms which do have nuclei.

The fact that we can't see them is a mundane fact about the fact that the wavelengths of light that our eyes detect are too wide to interact with them directly, but we have perfectly good reason to think they still exist. You have some recent work on mathematical realism, if I'm not mistaken. Yeah. So are you an ontextroman? structural realist. That term is used in such a tangle of ways. I have a view on realism that is the view I came to from reading various people who at various points describe them as themselves as ontic structural realists. The extent to which that view fits the views that various time slices themselves have held, I couldn't tell you. Some of them have been quite sympathetic

to it. So yeah, I mean, that ontic structural realism had been used to mean anything from the kind of very deflationary math-oriented metaphysics I like to a sort of priority monism with deep ties to scholastic metaphysics. Emily, what is your metaphysics?

I try not to be too committed to any specific metaphysics, but definitely broadly speaking, I am a structural realist, I guess. I think I was initially more attracted to an epistemic versions of structural realism, but more recently, partly as a result of thinking about relational approaches, actually,

I have moved more towards the sort of ontic end of the scale. So I guess broadly speaking, ontic structural realist, but I try to remain at open now. So you both agree in basically every way. No. You disagree, for instance, about whether on the current scientific evidence, we have excellent reasons to believe we're living in one of a vast number of power. So, Emily, what is scientific intersubjectivity? It's a large word, but what does it mean?

Why is it important to you? Why do you think that throws a monkey wrench into Everett? Why does a good physical theory have to secure it? Yes. So the reason I became concerned about intersubjectivity was through thinking about the measurement problem because there's a sort of general class of interpretations of quantum mechanics. not necessarily including Everett but adjacent to Everett, where there seem to be problems for intersubjectivity in the ordinary sense.

The most distinctive of those ones I've talked about the most is relational or quantum mechanics because the standard version of relational or quantum mechanics says that there are no absolute observer independent facts, and what that means is that there are no absolute facts about the relations between different observers, so it sort of can't be true in a non-relative sense that when I speak to you, what you hear matches what I expect myself to be saying. And so we can't sort of say that in any sort of observer independent sense. We're really succeeding a communicating information to one another. That I think is a problem for me because my understanding of the way in which we do science,

that involves a lot of processes that are very significantly based on the expectation that we can share information in an absolute sense with other scientists that I can do observations and tell you about them and then you can really learn what I have really seen and then we can arrive at a sort of shared reality within which we're doing science and arriving at conclusions. Relational quantum mechanics and various other sort of observer-dependent approaches doesn't seem to allow that. There's no shared world which sort of really exists in a way that goes beyond the perspective of any observer. It seems to me that I share a world with you. It seems to you that you share a world with me that neither is really right in an absolute sense. So my worry is that that is going to

undermine various kinds of roles that intersubjectivity plays in scientific practice, particularly the sort of special objectivity that comes to science and that, you know, the fact that there's lots of people working together and sharing ideas and critiquing and so on and so forth. So for that reason, I'm pretty skeptical about accepting any interpretation which says that we can't really share information in an absolute sense. And David, you hear that and think what? That I completely disagree. I mean, in the sense, I haven't got a dog in the fight. I don't, I mean, the version of Everett, I'm interested, doesn't have this problem particularly. But to defend the honour of the relationists, I mean, the, so, you know, and somewhat analogous

issues come up in the epistemology of Everett as well. The crucial place is how much we need to inject the phrase in an absolute sense into things. So do we need intersubjective agreement between scientists? Sure we do. Do we need to be able to trust that when I report my results to you, you understand the results the same way I gave them to you, yes, of course we do. Do we have to assume that the scientific community can communicate among themselves effectively? Yes, of course we do. Do we need to accept any of those things as being true when we insert the phrase in an absolute sense? I want to claim the former is just obviously necessary by just an internal observation of scientific practice, and the latter is something the philosophers put in

that goes beyond what's inside a sort of practice-based analysis of science. So if you look at the first paper Revelli ever has on relationism, it says something like the presumption we're going to make is that all statements are to be evaluated, anything you say, any statement at all in quantum mechanics or derivative of anything else, is always going to be evaluated relative to a context. So if you think that science requires agreement in an absolute sense, and if in an absolute sense is to be taken as explicitly metaphysically contradicting that statement, then yeah, sure, of course you better reject relational quantum mechanics. And we don't need to get into the weeds of relational quantum mechanics to establish that. It followed from the first sentence description of it. But I'm concerned that those reasons are question begging. I mean, I'm worried that they trade on an intuition we have about how we like the world to be. rather than on anything that's actively required.

I don't see any reason we can't, but from any given perspective, we can't tell a completely good story about how the scientific method plays out, including, from my perspective, I can say that from your perspective, these things match my perspective, the whole thing that commutes down.

The only thing you can't say is, from the absolute God's eye perspective, every perspective discreet. Because that statement is just not, you know, not grammatically, even really inside this sort of a relation to the query. It's kind of a bit analogous to ideas about the sort of nature of time. If you were to say, look, I insist that scientific facts are always true.

Then that's a clear necessity that various scientific assumptions have to be always true. We can't assume that they were just true sometimes and then stop being true later. But if you'd say that scientific claims must be true in a tenseless fashion, then that would be smuggling in a piece of metaphysics that might be defensible on metaphysical grounds, but which would be going substantially beyond the sort of internal to scientific practice analysis. I subscribe to the Economist.

Their science and their AI coverage is among the best I've found anywhere. And I say that as someone who reads plenty of it. I'll give you some examples. They just ran an analysis on how attitudes towards science are changing in American politics and what this means for research and funding in scientific institutions moving forward. This sort of high-quality reporting is fantastic.

They even covered how dark energy may be weakening over time. Now, if that holds up, it completely changes our understanding of the universe's fate. If you watch this channel, those are exactly the kinds of questions that we explore every week. I subscribe to the economist because their science and their AI reporting regularly surprises me with how deep it goes. and they're also, of course, known for global affairs, both political and economic reporting. They are top tier, and interestingly and flatteringly, Toa is one of the only podcasts that the Economist partners with. So as a listener, you get an exclusive 35% off. That's not a deal that they have just anywhere. Head to Economist.com slash TOE to subscribe. That's

Economist.com slash T-O-E for 35% off. Yeah, I mean, I think part of this comes down to a sort of internal tension in the way that relational approaches are sometimes formulated, because agreed the kind of philosophical setup is supposed to be that all statements are assessed relative to a context. But nonetheless, in many settings, the picture that the relationalists seem to want to suggest does involve sort of making a, non-statement, which is not indexed to anything about the existence of a whole bunch of perspectives such that from each perspective, quantum mechanics is true. And that is kind of the picture of the world, which is particularly motivated, for example, by the desire to make sense of what's going on in the witness frame scenario while allowing that everybody is right in what they have really seen. And my concern is that if that's the picture of the world that you're

trying to promote, then there is an internal inconsistency there in supposing that we have all these different perspectives, obeying quantum mechanics, and yet by construction, I can never find out anything about what's going on in any other perspective. I've got no reason to believe they have a quantum mechanics. So I think, you know, there is, I think, a consistent formulation of a relational approach, which really does commit to this view that you can't make that statement about the world as a whole. You can't talk about all the different perspectives. You can't talk about them being quantum mechanics. That, it seems to me, is not what many of the relationalists want and is not what is sort of primarily motivated by what's going on in quantum mechanics.

So while that's a possible way to make the view look consistent, I think that's not, there's a tension in the way they're formulated where they kind of want to have both things together. I think I can't have both. I think that's fair. I mean, to tie that back to where we were previously, try this. I mean, I'm saying I think the relationship should commit to the bit. But I think if they do, they probably get to where we're talking about earlier where relationism becomes an extremely close relative of the average interpretation. and these are to accept a whole bunch of things that you find unacceptable and the many relationships don't particularly want to buy either. Right. In denying their inner Everettianess, the relationshipists

run exactly into the problem you're describing because I do agree that these hybrids are very difficult to sustain. Yeah, right. So I guess I see it as a sort of dilemma where on one route, they do have this kind of hybrid story and then that seems to be inconsistent

for these endosurisendipity. reasons. The alternative is to, as you say, commit to the bit, but then that seems to fall into the problems I have whatever it. So I worry that neither of those routes is going to satisfy me personally. One person's delamory. One person's delamory. A question that the audience has on almost any discussion on many worlds is, look, if every outcome occurs or every possible outcome occurs, then what should they do? What should they care about? David, how do you, have you ever gotten a question like that from your students?

I'm not only if I got that almost all the time I've assumed What's your response to that? How do you view that? Yeah, I mean, there's a difference between things that happen due to really low probability mechanical events that still happen in certain branches

versus things that happen as a consequence of things we naturally analyze as my choices. So is there a branch where I choose to I don't know, pick up this laptop and smash it on the ground, let's say. No. Because I think my decision not to do that is like a really obvious and boring and clear act of my rational process is playing out. Is there some kind of neurological malfunction in my brain, which would cause a whole bunch of new, very low probability, cause a whole bunch of neurons to fire that would cause

my arms to move in such a way that I pick the laptop out and smashed it? Yeah, sure. I mean, almost certainly. And therefore, yeah, that's assuming some. incredibly low weight branch in which that happens. But it's not as if there's some sense in which I chose that to happen. And if you're going to find Everett viable anyway, and this is exactly where Emily wants to get off the bus, then you're going to have to have some reason to be unconcerned about even extremely bad things happening in exceptionally low weight branches. Or more or less generally you have to be committed to a kind of strategy that, crudely speaking, can treat exceptionally low-weight branches

as the kind of thing I just don't need to worry about. And there are plenty of deep and interesting philosophical issues as to why you should make that commitment. And again, if you're sufficiently convinced of your priors and you don't think it can be done, then you might reject Everett entirely on those grounds. But conditional on accepting Everett in the first place, you're going to be committed to the idea

that you don't much care if bad things happen in incredibly low-weight branches. So there's no particular reason, I'd say, to care differently, or in some qualitatively different sense in Everett, at least. Emily, does this have any bearing on your Costanza observers? Do you know what I'm referring to?

Oh, Jacobs Concerns Are Observers, yes. Yes, this is a reference to a television show, which I've never seen, so I can't explain the name. But the concerns are observers are precisely supposed to be the low-weight ones, and in particular observers who are, I think, reasoning in ways that are not rational at all or not rational by any sort of constraints that we would consider it at, consider to be normal constraints of rationality,

but who nonetheless end up being successful because unlikely things keep happening on their branch. So the concerns observers were supposed to be an objection relating to the idea that there are sort of plausible constraints of rationality in every single universe because there seem to be some kind of confounding observers who act irrationally and nonetheless succeed. And if you don't know, you're not one of those ones, why should you be rational?

So the joke, just for those who are watching and are wondering what the heck a Costanza observer is, my understanding is that it's referencing Seinfeld where George Costanza in one of the episodes does the opposite of what his mind tells him to do, so he's going to go against reason, and it actually works out for him. And much of the time is just by blind luck. And Emily, I think you were saying that in the Everettian, I don't know what to call it, interpretation, in the Everettian picture, in the Everardian picture, there are some branches where you as an agent can reason badly and everything still, what is it then? Everything still works out. It's not

exactly that. Sorry, what was it? Yeah, well, because there's some sense, there's some branch, admittedly with very low weight in which basically anything will happen in the Everett's story. There's going to be some branches in which observers are doing things we would consider quintessentially irrational, but where they are, nonetheless, wildly successful in all their endeavors, and it all works out very well for them. And so the thought was that the existence of such observers is supposed to put pressure on the idea that constraints of rationality can push us in the Eversian story to assign certain, to treat the weights as credences or as probabilities in certain senses because the existence of those observers in the first place is going to undermine any constraints

of rationality you might put forth. That's a strange line. I mean, you know, it can't be just qualifying of a theory that it allows for people who behave very irrationally, but nonetheless things work out with them. There are actual people like that. Some of them are in high office. Yes, that's true. I think the thought is that in the usual world, we would expect, maybe it would be

being optimistic, but we would expect that if you act irrationally a lot, you will be, you know, punished for your crimes against rationality and things will go badly for you, at least with some high probability. And the worry is that if you don't have a pre-existing probability style interpretation of the brunt weights, then you don't have any reason. to sort of rule out those cases or think that they're unlikely in any sense. And so therefore, it's not clear why you would expect any constraints of rationality to be relevant to you.

But it's a strange, I mean, look, this is getting into the weeds of the rationality stuff, but I mean, at some level, it's not quite clear even what it means to say constraints of rationality are not available to you. I mean, the way decision theoretic representation theorems generally work is not that they give positive arguments that some specific strategy is irrational. They just establish that, you know, they establish a class of strategies such that if you're outside that class of strategies, then you're committed to things that you will yourself accept a irrational, eG, algebra-level things, although it can get sophisticated.

So the form of those results is a lot more, you know, categorical than that. I mean, is it the case? Should one then have pause in that framework of the fact that the agents who are irrational and get lucky? Well, I mean, again, you've established that you don't much care about them because of how low their branch rate is. So, I mean, I'm not directly familiar with the form of the objections I'm not between Jacob's position justice, but that's part of the form of the concern. And look, it's also true that in any sufficiently large class of observers is going to have features like this. Even our actual worlds like that, I mean, people buy lottery tickets.

This is irrational, but some of them win and get very rich. It's always good, and it's not as if there's a, well, I mean, you'd be sympathetic to this. It's not as if there's a good probabilistic argument to say that I'm not one of them. I mean, I might be able to say that I, you know, but my, for any given lottery ticket I'm holding right now, I can say I'm very unlikely to win, but if I just sort of sit back and think what I think about the fact that I, share the world with observers some of whom are going to do irrational stuff and get lucky from it. I mean, good for them. Yeah, no, I think the concern was more that if you take a Dutch book style argument, for example,

the Dutch book argument is predicated on the thought that certain types of behaviour are irrational because they will deterministically or with very high probability lead to bad losses. The thought is that in an Everett-style universe, if you take these to the fabric branches seriously, then perhaps the Dutch book argument should not have the same force because you could act in this irrational way and then there could be just some weird freak event and you won't be given, you won't lose money even though you technically should give in it the way the bet was set up. And the thought is that if you don't have some prior reason for ruling out that kind of possibility or seeing it as very unlikely or having a reason to disregard it, then you don't get to use these

constraints of rationality and then you can't use them to constrain the branch ways to act in a way that would give an argument for the probabilistic treatment of them. I see. But, I mean, it's not obvious that the, I mean, I appreciate this is in your own argument, but it's not obvious the idealisation is any more unreasonable here in the Everett case. I mean, a Dutch book deterministically cause you to lose money. It's not just like with that probability. Yeah.

Now, could you, does the realisation of a Dutch book in the real world require some idealisation? Yeah, of course it does. But that's true for Dutch books in the none Everett in case as well. Yes. you can certainly, I don't think it would be difficult to get a Dutch book at the level such that if you assume no intervention from outside, you can get the probabilities down to the point of which we're talking about fluctuations or something rather than just like low-weight ordinary probabilities. Yeah, I think this comes down to a question about what sort of justifies

the use of idealisations of various kinds. And I think in the background of this argument is I thought that what justifies the use of idealisation is that with high probability, what really happens is going to be like close to what the idealisation suggests. And so the worry is that if you don't have that probabilistic concept sort of in at the get-go, then you don't have a good reason to think the idealisation is telling you anything useful and so you shouldn't be using it. Okay. I mean, dialectically, I think that objection lives earlier or should live earlier. I mean, if you, you know, there's an established set of arguments that people like Tebow and Dave Baker have used that's basically saying, look, the entire appeal to decoherence as a way of

understanding branching structure presupposes that we can use the mod squared amplitude metric as a metric of approximation. And that assumption, it's claimed, relies on the probability interpretation and so there's circularity. I mean, that's, I think, an interesting objection. I don't think it ultimately succeeds, but the reasons are subtle and fairly contestable. I sort of think by the time, if you've already got to the level of which you've got a branching structure and discussing decision strategy, you must have solved that problem anyway.

So I'm not sure that the problem could recur at that level. I mean, that's probably just to say if somebody had that can stand a concern, they should probably just move their objection back to the branching structure in the first place. Yeah, I mean, I think there's a real sense in which the branching structure is, can be extracted out in a way that doesn't necessarily depend on an idealization. but then there's a question about whether it can be given any particular kind of interpretation in terms of what we should expect to experience or see. And so I think in some sense, those two objections are closely connected to each other.

They're both trying to understand at what extent are we justified and interpreting this mathematical structure in a certain way. Yeah, I can. Kurt here, note that if you'd rather listen to Toe, we're on Spotify, iTunes, everywhere with a podcast catcher. You can just search my name or theory. of everything, and also remember to hit subscribe. Can I ask why you're not a cubist?

I don't think it succeeds in being a coherent explanation of quantum mechanics. I think it's an interesting exploration of how to think about various stylized quantum systems in the context of quantum information. It lacks the resources at present and doesn't have an obvious route to get them to describe most of the actual applications that we make of quantum mechanics. So if I want to understand the EPR experiment or Bell inequality violation, locality, maybe you're prepared to accept that cubism does that. If you want to understand why helium four and helium three are superfluid at different temperatures, cubism's got, which is an inherently quantum mechanical thing,

tubism has got nothing. So my basically requirement of interpretation of quantum mechanics, nothing much, you should just explain all of known physics. And Cubism is very badly short of that. It tries to explain the abstract quantum formalism, but doesn't succeed or really try to explain any of the concrete realizations of that formalism. That's my concern with them. Yeah, and I think there's a central core of a good idea in cubism, because I think it's based on the observation that quantum mechanics is a formalism

that we have designed for ourselves to use in certain contexts. takes, and of course that's true. One should take that into account of thinking about quantum mechanics. I think cubism just takes it to an extreme that becomes incoherent. You know, I think the case I like to think about, and a central commitment of cubism is that two different observers can assign different quantum states to the same system, and neither of them can ever be judged right or wrong. And in particular, two different observers, there's no restrictions on that so they can assign orthogonal states to the same system, and neither of them can be right or wrong. And so if you imagine a situation where you and I assign different states to the system, you might think we could just

measure the system and find out who's, who's right. The cubists can't allow that that procedure will allow us to determine who was right. So either they have to say it's impossible for us to talk to each other and find out what the measurement said. So that leads down the root of all these inter-subjectivity problems. The alternative is to say that we can do the measurement and we can both share the results, but we're just not allowed to use the result of the measurement to conclude anything about what the correct state of the system was. But if you say that, then you kind of cut off the means that we have to make empirical connections between the formalism of quantum mechanics and observed results.

You just wouldn't be able to use any observed results to conclude anything about what states are correct or whether your evolution equations are correct or anything that you might want to try to verify about the formalism. So either way, they just seems to be really serious problems understanding how the tubus view can make sense of the connections between the actual, observations we have made that have led us to arrive at quantum mechanics and the formalism itself. How good do you think that core good idea really is? I mean, yeah, sure, we came out of quantum mechanics as a formalism to describe our observations, but we came out of classical mechanics

did as well. I mean, obviously it's always an open possibility that whatever formalism we came up with encoded our experience. And maybe the point is that there's somewhat more hints of that in quantum mechanics than classically. Yeah, I mean, I think the cubists are right to draw attention to the fact that quantum mechanics has been designed from a standpoint and incorporates various aspects of that standpoint. And that's an important thing to think about and interpreting it. And because I think relational approaches up also, well, I think the idea that states are in some sense relative to a context or to an observer or something has some merit. But I think taking it to the degree where there are just no facts at all about what the right states are and nobody can either

correct anyone else about that, I think that that is obviously. obviously going too far and it doesn't do justice to the way in which the formalism is actually used. I see. I guess my worry is just, I'm reluctant to have this, so if there's any good inference just from the fact that something was designed from our standpoint to anything very much. Again, I mean, general relativity was designed from our standpoint, but I don't feel desperately inclined to have an epistemic interpretation of gravity waves on those grounds. Yeah, and I don't necessarily think that's a justification of systemic interpretation. I think that's a justification for perhaps thinking more carefully than we

have in the past about the ways in which our embodied standpoint might have influenced the way in which quantum mechanics has been formulated. Okay. I mean, maybe more sympathetic than I am to the fact that we should indeed do that for classical mechanics, which is fair enough. A few months ago, I was speaking to philosopher J.B. Mancheck. He's a philosopher of general relativity. And then he was telling me about the difference in style between himself and his advisor, Malament. That had me thinking about the differences in style between philosophers of physics in general, not just in GR. So I'm curious, how is it that you all see your own style? How do you tackle problems? How do you decide what counts as progress, et cetera?

Gosh, well, I guess the style that I do is partly informed by the fact that I was originally a physicist. I mean, my PhD was in physics, and so, you know, I think my motivation has always been similar to the physicists. And so the philosophy that I do, even when it is on quite philosophical topics, I think for me, is motivated by wanting to understand what's going on with the physics and have bitter ways of sort of talking about what's happening with the physics, with the hope of sort of being part of the advance of physics. I think there are other approaches to the philosophy of physics, which are much more interested in taking what we know about physics and using that to answer philosophical questions that have been around for a long time or get a new view on

those types of things. Whereas, yeah, I think I'm more on the side of trying to use the philosophical styles of thinking to clarify ideas about physics. So I guess that's one difference to some other types of ways of doing the subject. David. My approach to philosophy physics is really extremely conservative, is perhaps the right starting point. A quite large fraction of philosophy of physics is people coming in who for one reason or other are dissatisfied with various aspects of the mainstream in physics and in various ways want to push against it. And you see that again and again again. You see the widespread support for modifying quantum mechanics to solve the measurement problem or for minority approaches in

quantum gravity or minority. approaches in statistical mechanics or mathematical reforulations of quantum field theory or statistical mechanics or et cetera, et cetera, et cetera, et cetera. I'm very keen on contemporary physics and very keen on trying to approach it with a much stronger sense of trying extremely hard to make sense of it as is. Partly just because I think that's, ironically, a somewhat niche activity inside philosophy. I mean, this has changed a little bit, but I used to say that my specialty was the narrow niche area of mainstream physics. And I'm also interested in exploring, if I say the messy practice of actual physics, then colleagues of mine who are closer to labs and experiments will object. I guess I mean the messy practice of high theory.

So I'm interested in understanding physics very much at the abstract theoretical level, rather than at the level of the lab practitioner. But I'm interested in studying it at the level of mathematical rigor and approximation and tangle, if you like, that one finds in mainstream, you know, theoretical physics activity, whereas a lot of the trajectory in philosophy of physics has either been to go to the historical record and look at how these ideas developed historically or to look at extremely pure, precise, rigorous, mathematical codifications of these things

or to explore sort of dissident programs. I think all of those are, well, modulate some of the dissident programs. Most of those are extremely worthwhile things to do, but they're not the style of how I am interested in doing things. Are there any philosophical problems outside of physics that interest you? Both, David, we'll start with you. I mean, there's interest and interest, isn't there? I mean, many things interest me.

I'm interested in American politics and dinosaurs and planetary science and computer all playing games and so. In terms of my professional research interests, not a lot but not nothing. I mean, some of the stuff you were talking about about scientific realism goes a little bit beyond core physics, although it's still very physics grounded. I got involved in various questions in decision theory off the back of the workhouse during the ever interpretation, and I have some level of interest in those questions, even outside there. Many worlds context, and I've done a little bit of published work in those areas.

I'm sort of on the edge of having research interest in philosophy of mind, though again they sort of flow into my physics and structural realism interests. Yeah, I mean, somewhat similar. Yes, there are many areas of philosophy that I am interested in that I'm not competent to work in. I mean, I do work in a variety of areas of philosophy outside of strict philosophy of physics because they end up kind of bearing on things I'm interested in in the philosophy of physics. I've worked on the philosophy of lawhood because I'm interested in the way it relates to things I think about in physics with relation to the role of time.

And I worked on self-location because it ties into the everett interpretation and cosmology. And I worked on a few sort of philosophy of mind-style things because I think it is relevant to consider. terms I have about time and the experience of time in physics. So yeah, I mean, all of these things stem from the work that I do in physics, but I end up going a bit beyond it sometimes. Now, to close, as you mentioned, you all speak often to each other at great length, and you all have different styles and strengths. So what is it, David, that you respect about Emily's style and strength, or admire even, and Emily, respectively to David? goodness. I mean, I think there's an admirable intellectual honesty in recognizing how severely difficult various moves are going to be,

and not kind of being in denial to that and taking entirely seriously the physics problems there, while still having the courage of one's convictions to say yes, this is extremely difficult work, but nonetheless it has to be done because the arguments for it are compelling. Yeah, I think as David says, his work takes seriously physics as it actually is and tries to understand and clarify issues in it and have a better sense of what's going on. the physics and what that tells us about philosophy. I like that. I find it very helpful. I think I'm very clarifying to read that stuff and it certainly improves my own understanding of what's going on. I think I personally am more willing to alter things, but I think nonetheless is really important

if you've got to do that to really understand what's actually happening and why it's happening if I find David's work very helpful for that. Well, I admire both of you. And every time I speak with each of you, I realize how little I know and how much farther I have to go. and I feel like a fool, but I learned so much, and I appreciate you both tremendously. Thank you. Well, thank you.

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