The Many Worlds Interpretation (Everettian quantum mechanics) faces a fundamental epistemological challenge: if all possible outcomes occur in separate branches, it becomes difficult to justify using relative frequencies as empirical evidence for the theory, potentially undermining the very evidence used to support quantum mechanics. Additionally, relational approaches to quantum mechanics face a tension between their claim that all statements are relative to a context and the need for scientific intersubjectivity—the ability for scientists to share information and arrive at shared conclusions about reality. This creates a dilemma where relational approaches may either commit to the same metaphysical commitments as Many Worlds or face internal inconsistencies that undermine their explanatory power.
The Quantum Interpretation That Divides Physicists
Added:If you disagree about whether we're living in one of the vast number of palagans, >> it doesn't make sense to believe an 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 worlds interpretation. [music] Years prior, David supervised Emily's undergraduate thesis at Oxford. [music] >> I think it's much more likely we'll learn something about epistemology from resolving that than learn something about physics. Professor Emily Adlam, who's also been on the podcast [music] before, argued that no one inside many worlds could ever confirm many worlds.
Both believe the philosophy [music] of physics is just as important as the physics itself.
>> I don't know which interpretation is right.
>> On this channel, I, Kirchaung, 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 wave function? We'll start with you, David.
>> It's the representational tool in quantum mechanics that encodes the physical features systems have. So the there's a take on the wave function that just says it's epistemic, but the take I want to say it's it's it's not a thing in itself. That that's a category error, but uh it encodes the properties things have and the relations they stand in. And for the a given system, the wave function of that system just encodes all the properties that system has.
>> Emily, do you broadly agree?
>> Uh to some extent. I mean, I think there are two types of roles that a state sort of tends to play in physics and one is to represent the sort of occurren at a time and another is to encode predictions about how the systems are going to behave in the future. uh 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 the current properties at a given time. Um 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 it's whether we we should assume that the wave function represents properties or whether it just rather represents a sort of set 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 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 where 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. Um I'm a bit more dogmatic as to what I think the right interpretation is Emily. So I'm probably willing to be a bit more committal there. Um so I would say that to ask what the wave function is onlogically for everyone is kind of the wrong question to ask. it's not a thing or or at least to suppose it's a thing is a big substantive extra step. There's nothing in even very realist approaches to quantum mechanics to say that the wave function itself is a thing. Um 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 inferential reading of its right then it encodes something else.
But the point is it's not uh you know it's 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 that a universal wave function is can we talk about the universe as a whole as a quantum system with properties that can be represented. Um 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 wave function is a good question partly because uh 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 wave function because wave functions always have to be attributed relative to to something else. Um so that that I guess is one of the central points setting apart various relational approaches from the interpretation for example. Um uh and I think one one reason to be a bit skeptical about the use of a universal wave function is because the sort of central observation that the relational approaches want to make is that in practice we do always use wave functions relationally 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 wave 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 Aadian 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 your getting a passport there?
>> Um well, you know, >> where do you only visit?
>> Where do you even visit?
>> Do I even visit? [laughter] >> I have visited in the past. Yes. Um no, I I I could definitely see the appeal of the Emirates approach. not least because I think it's the only approach which sort of uh reliably works in all applications that we we sort of know of and so that's obviously a very appealing feature. Um the worries that I have about Everest are to do with the the role of probabilities in empirical confirmations. Obviously there's sort of long-standing 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 in conservation to make sense of quantum mechanics. So for that reason I I am doubtful and would like to see 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 ever is false? It's that Everett under some interpretation, I know there are different slices. There are different there's there's a Wallace view and and there's a 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. Like what are you precisely saying Emily? Yeah. Well, the worry is that if there is no adequate solution to the Everettian probability problem, then if you believe that you are in an Everettian world, it would follow that you wouldn't be able to use the relative frequencies and so on as empirical evidence for a theory in 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. Um, but it doesn't make sense to believe an interpretation of quantum mechanics which says we shouldn't believe quantum mechanics. So, the whole thing looks a bit self- undermining if if the probability problem can't be resolved.
So it comes down to can it be resolved?
>> Yeah. I mean in a certain sense I agree with that but I put the logic a little differently. I I mean here's the thing about Everettian quantum mechanics. The the framework of unitary quantum mechanics. So the mathematical core of what ever using which I'd say is the the standard equations of quantum mechanics but also commitment to the fact that measurement is a 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 cross the quantum and the classical is just a uh 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 um physics practice in doing that. And that's that's different I'd say from where we were in quantum mechanics in say prior to the 1980s pri 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 [laughter] of the of of the result there. I mean I think we're we don't have a lot of precedent of situations in which um we've got a completely successful scientific theory where we have some entirely external to it um philosophical conceptual constraint. I think if there if there is some kind of clash between uh 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 Ever, but that's my kind of starting point. I I think it's a I think it's an important philosophical problem. I think we can learn a lot about um probability from thinking about it. And dialectically, since a bunch of people are ever sympathetic but worried about probability, uh in so far as there are things that can be said to assuage those worries, it's worth saying. But um I would need a hell of an argument to convince me that on sort of our priority philosophical grounds um 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 just imagine that you all are at a conference and and I'm not here and you can also feel free to ask questions to one another.
>> Uh yeah. Well, I mean I I should say that I I also am not keen to reject quantum mechanics or any large part of the framework of it. Um you know, obviously the my ideal interpretation would be an interpretation that preserved quantum mechanics as is and yet was not the first interpretation.
>> [laughter] >> um and and you know obviously I don't I don't have a complete version of such interpretation to offer. So that is a problem. I mean because I I think that ever has this very significant problem uh and because I nonetheless think that quantum mechanics as it currently exists is is so well well uh demonstrated that gives me some faith that there must be another interpretation which would work.
But yeah, I think I I agree that currently you don't have such a thing and so we should be looking for it.
>> The interpretation of that formalism. So you wouldn't think something like an Adrien and Kent's proposals for photonbased final state theories would would would be as attractive. Just trying to look at the parameters.
>> I think because the way in which quantum mechanics put is put together is quite sensitive and quite difficult to alter successfully. Um an ideal interpretation should either change nothing or change as little as possible. Um so for that reason I'm I'm quite skeptical about sort of bone bone style interpretations or collapse style interpretations which require changing a lot. Um I'm hopeful that there might be ways to have other types of interpretations which are not ever but which nonetheless maintain very significant aspects of the existing formalism. That's one of the reasons why I'm 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. Um, it's actually let me back off. Sorry, I tangled my chain of thought. Um, let me just put since I paused, let me just say for Kurt, um, I'm play like we're doing the let's talk the conference framework, but Emily and I have talked about this stuff a lot.
[laughter] So, the the way we actually want to talk about it is inevitably going to have to have a fair amount of pedigogy. If we just have a direct conversation about this, I think it's going to go >> Oh, that's totally fine. Have a 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.
>> All right fine. Um so maybe distinguish between um modification light and pure interpretation. So there might be modificatory moves that as you say are less kind of thoroughgoing than dynamical collapse theories or hidden variable theories. I'm I'm really skeptical but if there were then yeah I can see that's a window you might be able to explore.
>> Yeah. Um and then there's kind of genuinely pure interpretation. So there's if there's a pure interpretation of QM that's not ever then yeah I'm happy to say that's very much on the table and we can talk about it. Um I mean from that point of view if we think about the relational idea I I guess the question is and I think this has come up in some of your work there's a there's a sort of tension where if you it's hard to find a happy medium that doesn't commit you either to a relatively thoroughgoing modification the formalism or basically claps back to Everett. I mean I'm not you if if you take the sort of original relli idea that the 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 you some some other system [sighs] I 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 I don't think there are zero cases of use that in physic but there ex exception few. So I'm I'd be 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 of that ever 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 the sort of thoroughgoing modification that as you say is going to run into a ton of technical problems.
>> Yeah. No, I agree with that. Um 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. Um, having thought about this for a while, it seems to be obvious that you can't. Um, this can't be done. Uh, the only thing you can really do is what was a recent paper by Relli and Dagio just basically proposing a version of Everest. Um, saying that you can look from the outside and you can see something that kind of looks like a measurement as the things become entangled and discussed this with them and said, isn't this just Everest? And the conclusion is, well, yes, essentially it's it has some different philosophical emphases, 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 uh which preserves the entire formalism of quantum mechanics and adds nothing to it. Um I'm still hopeful that there are directions you could investigate which would be relatively minor modificatory strategies and which might would would still leave quantum mechanics intact in the domains where it's successful. Um but that's obviously more a promisory note than a well-developed idea at this stage. Yeah, I mean that's fair enough and it's worth saying I mean look um I'm I'm perfectly pro people working on things like that.
I'm perfectly pro people working on dynamical plast.
Um I mean as a if if we're doing physics rather than philosophy then absolutely let's explore the theory space. Um but then I'm separately interested in the question well okay given the actual best theory we have at the moment.
>> Yeah.
>> Um what are we going to do with it? How do we think about it? Um >> I mean I mean just to just to clarify what you have in mind for the sort of relli my guess is we're doing something like the following. I I can be a relationist, but I can still say, "Okay, fine. You go and do the shredding a cat experiment and I retreat to Alpha Centuri because I'm distressed by violence to cats." Um, while I'm on Alpha Centuri and you do the experiment and you're in my 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 superp position of life cat and a dead cat."
>> Yeah.
>> Yeah.
>> Uh, yeah. I mean the question is is being posed in the literature is about you know how do you say when and how a measurement has taken place in in RQM and and the answer they want to give is that you look from the outside and you see that adequate correlations have come 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 um which seems to me more or less exactly what you would also say within a sort of Everett story. Um, so yeah, there's there's no there's no room in that story for a specific individual outcome to America.
>> Well, I mean, after all, the the the rebellion relative state is perfectly definable inside the Ever interpretation. Um, and as you as you point out, the the universal state is not doing a hell of a lot of work in that story, but the the state relative to me may be enormously indefinite, which >> yes, >> gets you the problems. Yeah.
>> So, I'm hearing an awful lot of I agree.
I agree. I agree.
>> Where's the disagreement? Well, I think the disagreement is that I, you know, despite seeing the the attractions of the ever 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', you think there are various directions to look in. Uh I think there are interesting new ideas coming in from more recent work in physics and in quantum gravity and so on. And so I'm sort of hopeful that there are other options that we just haven't explored yet. Um but yeah I mean I I agree that kind of the existing interpretations that we have ever it is the one that apart from these epistemic concerns sort of works best and is most easily applicable. So just to push your um your prior on this how you know how committed are you these epistemic things suppose we prove some frankly fairly implausible nogo theorem that says that no minor tweak of the formalism of quantum mechanics is um uh is going to get you away from the ever interpretation. you have to do a thoroughgoing modification of the formalism uh along the lines of anamical collapse theory or invariable theory um that that's the only way you could get around it. Are you still confident enough of those epistemic arguments you want to say nonetheless um what what we conclude from that is that it must be that there exists some fur modification of this kind.
>> Uh yes, I would conclude that. Yeah, I would be disappointed because that looks like very hard work. But uh I think the epistemic concerns are for me serious enough that you know unless someone comes up with a what an argument that I would find more convincing to show that the evering probabilities can play the right kind of role then just as a matter of kind of internal logic I just can't accept it.
>> So I mean let me push a little on that.
I mean yes obviously as a matter of internal logic if you have confided premises then you have to eject some.
But I mean, you know, I I love philosophy. Some of my best friends are philosophers. I got paid by my way to come from philosophy.
But >> does analytic philosophy really have the kind of track record to give one that level of confidence in uh epistemological prior? 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 uh and the only remaining obstacle was this was the epistemic concerns that might be a different situation. You know part of the reason I I am willing to investigate 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 roots to explore which might modify quantum mechanics anyway and and and which might potentially also give interpretations which I find more coherent.
>> Good. Okay, that's that's that's reason enough. So, so sure. So, yes, if you're inclined to think something 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 and conversely, I mean, uh I think string theory is very substantially our best strategy for quantum gravity at the moment. Um >> uh I think and that that's just a a unitary unmodified quantum theory. um modulates some slight delicacies about the problem of time. So is loop quantum gravity. So is astronomotic safety.
These are all the those are what I would count the serious runner up processes.
So I'm hardly moved by the fact all of those are quantum theories. So >> yes, although I mean I think I have some concerns [clears throat] 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 in formulating theories of quantum gravity and difficulties in interpretation of existing theory.
>> Sure. Yeah. I mean either way again I'm probably probably a lot more optimistic about how well we're doing than you are.
Um but no sure I mean I think I think it's um I mean look I 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 a pro frame rate.
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That's pl Aud.ai. AI/currt and use code cur kurtz for a 15% discount. This video is sponsored by plaude. 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. Um well I mean some of the inter 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 like tie into the theory. So I mean the 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. um despite the fact that it seems as though there's no sort of evolution in in some versions of quantum mechanics. Another issue is that quite a lot of uh in quantum gravity for example the most straightforward way to do a calculation is kind of like fix an external classical 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 uh in in much the same ways as you do in sort of uh standard non-relativistic 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 seems to me at least plausible there's some link between the problems we're having in quantum gravity and the other earlier problems that in interpreting quantum mechanics.
>> I'm not 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 basically temporally. And that's in classical GR likewise will will individuate our subsystems and locate them against the sort of classically specified background. We'll put conditions there. I think we have fairly satisfactory quantum theories of gravity at least in string theory. Um I agree we have no satisfactory quantum theory of cosmology and >> part of the reason for that I guess is precisely this system localization. I mean I'm slightly skeptical 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 quite odd uh in the point of view of uh certain kinds of conical quantization for example. Um so you know while I agree that you can look at an individual subsystem and you know derive useful predictions out of it quantum gravitationally uh the the the separation between those two problems itself seems to me to be a kind of approximation and something that we shouldn't sort of rely on conceptually.
>> Um I mean I could get in the weeds of quantum gravity. Let me really strongly object to the idea that um because something's an approximation we shouldn't rely on it conceptually.
>> I mean everything is an approximation.
[laughter] Natural selection is an approximation. I still want to use it to explain adaptive complexity.
>> Sure. No, that's fair. Now, we can we we can use the approximation if our goal is to understand how to make predictions for the quantum gravity of some effectively isolated subsystem. Um um 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 uh links to and h to uh earlier frameworks and how uh our experience and quantum non-authentic quantum mechanics merges out of it. I'm slightly skeptical that you can tell that story in a 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 maybe are not in earlier physics.
>> How do you think that goes? I mean that's not obvious to me. If I take a an isolated system um I put it in a box. I idealize the box in certain respects. Um I do quantum gravity inside the box and I look at the degree and in some regimes I'll get a big black hole and I get interesting quantum quantum mechanical stuff on the surface of the black hole.
In other regimes I'll get perturbations of flat spacetime and I can just do QFT.
Um I mean that's the regime in which we recover low energy quantum gravity and quantum field theory from well as it were string theory is the place where that's most concretely successful. I 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-cossmological and strong gravity versus non-strong gravity bits don't commute.
Um well I mean for example from a relational point of view again um one of the sort of recent uh directions of investigation in the relational formalism has been looking at the way in which subsystem decompositions depend on a choice of reference frame. Um so you know given a fixed subsystem decomposition you can for sure do a description of it and describe the physics of just that subsystem uh sort of relative to the choice of background.
Um but it seemed to me that if you want to tell a complete story about how all that is emerging from whatever is underneath then you are going to have to take into account the fact that the subsystem decomposition itself is not fixed and then that's and that the procedure for choosing it is part of the story.
>> Okay. Yeah. Yeah, I mean I could believe that that might depend a bit on how much the relation we're using. And I suppose I'm just concerned that we do I mean in some ways I'm reiterating 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.
Um even while recognizing that the cosmological situation is still much more opaque.
>> Yeah. No, that's fair enough. Yeah. And I think I I agree that we can do a lot of things usefully, but there are also sort of certain questions that seem to be important about the way these things are tied together.
>> Yeah. Yeah, that's fair. Okay.
>> 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 weight 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 is 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 if push comes to shove and a large modification is the only thing that will work then I I'm willing to do that. I'm willing for someone else to do the work. Um but [laughter] I think it pro I would be I would be inclined to think it has to be done in that case.
>> So the place where where I slightly disagree Kurt is um 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. Um and you arguing for that was the main main theme of the earlier phase of my intellectual life. Um what I was saying in this context is that even persuade me that what I see as satisfactory philosophical solutions here don't work. Um 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 um to uh to think we should abandon that formalism and and to some extent I mean at that level it becomes becomes inevitably inconclusive. I mean there's a matter of one's sort of prior as to what's likely to break which is kind of what we're talking about. There's also a matter of division of labor. I mean my and I don't 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 formulas of quantum mechanics, this is not something within the skill set of the typical even highly technically minded philosopher of physics. Um 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 um the more sort of flat flatly let's um uh let's let's look 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 decent. So, do you all dislike say dynamical approaches or a a Penrose approach or a Tim Modellian 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? [laughter] 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 alternative of quantum mechanics and theories that are attend 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 of the wave function shown as cat resolving into alive or dead but not both is something that's really supposed to happen as a matter of micronamics.
um 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 that you want to think of those theories as ways of parameterizing the space of what it what what 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 of those theories in the forms we have now is to let us do experiments um or or work out what experiments we could do and try to test those theories.
Um I mean I think Clap's theories are good science and one of the ways to tell they're good science is that I think they're within 10 or 20 years of being completely ruled out experimentally.
[laughter] >> 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?
>> Uh well only if it's true Everett's false. [laughter] um if if on Oppenheimer's approach then um uh effectively you have wave function collapse. So if um if I can have a superp position of the matter the the idea of the Oenheim approach and it's it's it's 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. Um if the quantum superp positions evolve unitarily and don't collapse that will rapidly and radically violate experiment. Um 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 superp position 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 it's not really that smeared anymore. Um so every obviously if that's right then these macroscopic superp positions whichever it's committed to don't exist.
>> Yeah. Yeah, I mean I think that those styles of approaches which maintain sort of a classicality of spaceime and gravity are are appealing because there are a bunch of both conceptual and technical difficulties that come from trying to make sense of superp positions of spaceime. So I can see the appeal of avoiding them. Um and you know I think that the technical challenges of making such a thing work are strong. Uh, and I would probably have low credence that it will work in the end, but I think because it has such nice features that there's a good reason to explore it. I >> I mean, I'd say I'm I I think the technical problems of having superpositions of geometry are massively overstated. Um, I actually think the technical problems of not having them are much more severe. I think we have a mathematically fairly satisfactory theory of quantum gravity at energies below the plank scale. Well, below the plank scale.
>> Yes. Um there was some you know it has a whole bunch of interpretational questions um some of which are just the measurement problem but you bracket that.
>> Um so I'll so I mean yeah I hesitate to disagree with Sir Roger Penrose but I do what I you know it's worth saying Pen is very much has a minority view here.
>> Um however again in 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 I have money riding on the fact that it will fail the test and be eliminated. [laughter] >> I would be very happy to lose the bet because it would open a amazing vista of new physics. We'd learn a ton of stuff.
Um I'm not expecting to learn those things, but the point is will um the 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 either.
>> Is it a poly market bet?
>> Uh, no. No, it's just a friend of mine who believes in collapse theories, which one of us has to buy the other one a I forget. Bottle 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.
[laughter] >> [gasps] >> Realism very loosely is the picture that our scientific theories are telling us things about what the world is like. What or perhaps more mundainely 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 the scales beyond those senses. So classically speaking, the 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 up 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 antic structural realist?
[gasps] >> That term is used in such a a tangle of ways. Um I have a view on realism that is the view I came to from reading various people who at various points described them as themselves as structural realists. Um the extent to which that view fits the views that various time slices themselves have held I couldn't tell you. Um some of them have been quite sympathetic to it. So, so yeah, I mean that on structural realism has been used to mean anything from the kind of very deflationary math oriented metaphysics I like to uh a sort of priority monism with deep ties to slastic metaphysics.
>> Emily, what is your metaphysics?
>> Um, I try not to be too committed to any specific metaphysics, but definitely broadly speaking, I am a structural realist, I guess. Um I think I was initially more attracted toward epistemic versions of structural realism but um more recently partly as a result of thinking about uh relational approaches actually I have become I've moved more towards the sort of end of the scale. So I I guess broadly speaking on structural realist realist but I try to remain a bit open there.
>> Well so you both agree in basically every way.
No, [laughter] >> we disagree, for instance, about whether on current scientific evidence we have excellent reason to believe we're living in one of a vast number of power unions.
[laughter] >> 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?
>> Uh, yes. So um and the reason I became concerned about inter subjectivity was um through thinking about the measurement problem because there's a sort of general class of interpretations of quantum mechanics uh not not necessarily including Everett but adjacent to Everett uh where there seem to be problems for inter subjectivity in the ordinary sense. um the the most distinctive of those ones I've talked about the most is relational quantum mechanics because the standard version of relational quantum mechanics says that there are no absolute uh 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 uh what you hear matches what I expect myself to be saying. And so um we can't sort of say that in any sort of uh observer independent sense.
we're really succeeding at communicating information to one another. Um that I think is is a problem for me because my understanding of the way in which we do science that involves uh a lot of processes that are very significantly based on the expectation that we can share information in a in a absolute sense with other scientists that you know 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. um relational quantum mechanics and various other sort of observer dependent approaches doesn't seem to allow that um that there's no shared world which sort of really exists in a way that goes beyond the perspective of any any observer. It seems to me that I share a world with you. It seems to you that you share a world with me about neither of is really right in an absolute sense. Uh so my my worry is that that that is going to undermine various kinds of roles that interubjectivity plays in scientific practice particularly the sort of special objectivity that comes to science in virtue of the fact that there's lots lots of people working together and sharing ideas and critiquing and so on and so forth. So for that reason I I'm pretty skeptical about accepting any interpretation which says that we can't really share information in absolute sense.
>> And David you hear that and think what >> um that I completely disagree.
[laughter] I mean, in a sense, I haven't got a dog in the fight. I don't I mean, the the version of Everett I'm interested doesn't have this problem particularly.
Um, but to defend the honor of the relationists um I mean the so so so you know some and somewhat analogous issues come up in the epistemology of Everett as well. Um uh the crucial place is how much we need to inject the phrase in an absolute sense into things. So do we need interjective agreement between scientists? Sure we do. Um uh do we need uh to be able to trust that when I report my results to you, you uh understand the results the same way I gave them to you? Yes, of course we do. Do we have to assume that uh 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? Um I want to claim the former is just obviously necessary by just an internal observation of scientific practice and the latter is something that philosophers put in um uh that goes beyond what's inside a sort of practice based analysis of science. So if you look at the first the first paper RLI ever has on relationism, it says something like the presumption we're going to make is that all um all statements are to be evaluated anything you say any any any statement at all in quantum mechanics or derivatively anything else is always going to be evaluated relative to a context. Um 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 um 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, um, I'm worried that they trade on an intuition we have about how we like the world to be, um, rather than on anything that's actively required. I don't see any reason we can't that from any given perspective, we can't tell a completely good story about how the scientific method plays out. Um, including from my perspective, I can say that from your perspective, these things match my perspective. The the whole thing sort of commutes down. Um the only thing you can't say is from the absolute god's eye perspective um uh every perspective disagrees because that statement is just not you know not grammatically even really inside this sort of relation of creation. It's kind of a bit analogist to ideas about the the sort of nature of time. If you were to say, look, I insist that scientific facts are always true.
Um, then that's, you know, there clear necessity that various scientific assumptions have to be always true. Um, we can't assume that they're just true sometimes and then stop being true later. Um but if you'd say scientific claims must be true in a tenseless fashion um then that would be smuggling in a piece of metaphysics that might be defensible on metaphysical grounds but which wouldn't would be going substantially beyond the um the sort of internal to scientific practice analysis. I subscribe to the economist.
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>> 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. Um because agreed uh the kind of philosophical uh setup is supposed to be that all statements are assessed relative to a context. But nonetheless uh in many settings the picture that the relationalists seem to want to uh 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. Um and that is kind of the picture of the world which is uh 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. Um, and my concern is that if that's the picture of the world uh that you're trying to promote, then there is an internal inconsist inconsistency there and supposing that we have all these different perspectives surveying 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. Um, so I think you know there 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.
can't talk about in quantum mechanics.
Uh that it seems to me is not what many of the relationalists want and is not what is sort of primarily motivated by uh what's going on in quantum mechanics.
Um so uh while that's a possible way to make the view look 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 and 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. Um I mean I'm saying I think the relation should commit to the bit. Um But I think if they do, they probably get to where we were talking about earlier where relationism becomes an extremely close relative of the average interpretation.
>> Yeah.
>> And need to accept a whole bunch of things about Ever that you find unacceptable. And then many relationists don't particularly want to buy either.
>> Um, in denying their in Everettianness, the uh relationists uh run exactly into the problem you're describing because I do agree that these hybrids are >> very difficult to sustain. Yeah. Right.
Uh so I guess I see as a sort of dilemma where on on one route they do have this kind of hybrid story and then that seems to be inconsistent for these indogitivity reasons. The alternative is to as you say commit to the bit but then that seems to fall into the problems I have with Everett. So I worry that neither of those roots is going to satisfy me personally. [laughter] >> One person's dilemma is another person.
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 have I got that almost all the time from students.
>> What's your response to that? How do you view that?
Yeah, I mean there's a 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 consequences of things we naturally analyze as my choices. So, um 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. Um no. Um 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 very low probability cause a whole bunch of neurons to fire that would cause my arms to move in such a way that I picked the laptop up and smashed it? Yeah, sure. I mean almost certainly. And therefore, yeah, that's presumably 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. Um, and if you're going to find Everett viable anyway, and this is exactly what Emily wants to get off the bus, um, then you're going to have to have some reason to be unconcerned about even extremely bad things happening in exceptionally low weight branches.
You're more or less generally have to be committed to a kind of strategy that crudely speaking treats exceptionally low weight branches as the kind of thing I just don't need to worry about. Um, 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 prior and you don't think it can be done, then you might get you might reject Everett entirely on those grounds. But conditional on accepting effort 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 lowweight branches. So there's no particular reason, I'd say, to care differently or in some qualitatively different sense in ever than in the non ever case.
Emily, does this have any bearing on your Castanza observers? Do you know what I'm referring to?
>> Oh, Jacob's Jacob's concerns are observers. Yes. Um, uh, yes. This is this is a reference to a television show which I've never seen so I can't explain the name but um uh the concern observers are precisely supposed to be the the lowweight ones and in particular uh observers who are I think reasoning in uh ways that are not rational at all or not rational by any sort of constraints that we would consider uh consider to be normal constraints of rationality but who nonetheless end up being successful because unlikely things keep happening on that branch. Um so the the concerns observers were supposed to be an objection uh relating to the idea that there are sort of plausible constraints of rationality in every universe because u that there seem to be some sort of 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 castanza observer is my understanding is that it's it's referencing Seinfeld where George Castanza 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 it's just by blind luck and Emily I think you were saying that in the everian I don't know what to call it the interpretation in in the everian picture in the everian 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 be because and there's some sense there's some branch adittly with very low weight in which basically anything will happen in the Everett story. There's going to be some branches in which uh 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. Um and so the thought was that the existence of such observers is supposed to put pressure on the idea that constraints of rationality uh can uh push us in the evolution story to assign certain to to treat the weights as as credences or as probabilities in certain senses because uh the existence of those observers in the first place place is going to undermine any constraints of rationality you might put forth.
>> That's a it's a strange line. I mean, you know, it can't be disqualifying of a theory. It just allows for people who behave very irrationally, but nonetheless things work out. There are actual people like that. Some of them are in high office.
>> Yes, that's true. [laughter] Uh I I I mean, I think the thought is that in the usual world, we we would expect maybe being optimistic, but we would expect that if you act irrationally a lot, you 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 uh probability style interpretation of the BR branch 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, there's 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 the it's not quite clear even what it means to say constraints of rationality are not available to you. I mean the the the way decision theoretic representation theorems generally work is not that they give positive arguments that some specific strategy is irrational.
>> Yeah. 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 acceptational egot things although it can get sophisticated.
>> Um so the for the form of those results is a lot more you know categorical than that. I mean I mean is it is it the case none should should one then have pause in that framework of the fact that the agents 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.
Um so I mean it's I'm not directly familiar with the form of the objection so I might be doing Jacob's position justice but that's that's part of the form of the concern. And look, it's also true that in any any sufficiently large class of observers is going to have features like this. I mean, again, even our even our actual world is like that. I mean, people buy lottery tickets. This is irrational that some of them win and get very rich. Um, it's there's always 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, my for any given lottery ticket I'm holding right now, I can say I'm it's 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, well, you know, I mean, good for them.
>> Yeah. No, I think the the concern was more that if you look take a Dutchbook style argument for example, the Dutch book argument is predicated on the thought that certain certain types of behavior are irrational because they they will deterministically or with very high probability lead to bad losses. Um the thought is that uh in an everate style universe if you take these sort of maverick 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 you'll be given you won't lose money even though you technically should given 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 weights and in a way that would give a argument for the probabistic treatment of them.
>> I see. But I mean it's not obvious that the I mean this is in your own argument, but it's not obvious the idealization is any more unreasonable here in the Ever case. I mean Dut a Dutch book deterministically causes you to lose money. It's not just like with probability. Yeah.
Does the realization of a Dutch book in the real world require some idealization? Yeah, of course it does.
But that's true. Dutch books in the non everettian case as well. Um uh you can certainly I don't think it would be difficult to get a Dutch book at the level that such that if you if you assume no intervention from outside, you can get the probabilities down to the point to 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 idealizations of various kinds. And I think in the background of this argument is a thought that what justifies the use of idealization is that with high probability what really happens is going to be like close to what the idealization suggests. And so the worry is that if you don't have that probistic concept sort of and at the get-go, then you don't have a good reason to think the idealization is telling you anything useful and so you shouldn't be using it.
>> Okay. I I mean dialectically I think that objection lives earlier or should live earlier. I mean if you you there's a an established set of arguments that people like Tebo and um 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 squareed amplitude metric as a metric of approximation. M >> um and that assumption explained uh relies on the probability interpretation and so there's circularity. I mean uh that's I think an interesting objection.
I don't think it ultimately succeeds but the reasons are subtle and fairly contestable. Um I sort of think by the time if you if you've already got to the level at which you've got a branching structure and you're discussing um decision strategy, you must have solved that problem anyway. Um, so it's odd to I'm not sure that the the problem could recur at that level. I mean, that's probably just to say if somebody had that constant 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 there's a real sense in which the branching structure is 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 uh in some sense those those two objections are closely connected to each other and they're both trying to understand to what extent are we justified in interpreting this mathematical structure in a certain way.
>> Yeah, I get 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 theories of everything and also remember to hit subscribe. Can I ask why you're not a cubist?
>> Um, 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 um various stylized quantum systems in the context of quantum information.
um 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 4 and helium 3 are super fluid at different temperatures cubism which is an inherently quantum mechanical thing cubism has got nothing.
So my my basically requirement of interpretation of quantum mechanics um you know nothing much. It should just explain all of all of known physics. Um [laughter] uh and cubism is very badly short of that. It explain it 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 that's my concern with it.
>> Yeah. Yeah, I mean I think there's a central core of a good idea in cubism because you know 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 and of course that's true. Uh one should take that into account when thinking about quantum mechanics. I think cubism just takes it to an extreme that becomes incoherent. Um you know I think the case I like to think about and a central commitment of cubis 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 uh if you imagine a situation where you know 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 and sort of agree on what the measurement said. So that leads down the route of all these interceptivity problems. Uh the alternative is to say uh that we can do the measurement and we can both share the result 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. If you say that then you kind of cut off the the the 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 anything that you might want to try to verify about the formalism. So either way, there just seems to be really serious problems to understanding how the cubist view can make make sense of the connections between the actual observations we've 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 up with quantum mechanics as a formalism to describe observations, but we came up with classical mechanics did as well. You know, I mean, >> I mean, obviously, it's always there always an open possibility that whatever formalism we came out up with encoded our um experience and maybe the point is that there are sort of somewhat more hints of that in quantum mechanics than classicalally.
>> Yeah, I mean I think the cubists are right to draw attention to the fact that uh quantum mechanics has been designed from a standpoint and incorporates various aspects of that standpoint and that that's an important thing to think about when interpreting it. Um uh and you know I I because I think relational approaches are plausible. think the idea that uh states are in some sense relative to a context or to an observer or something has some merit. Uh 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 ever correct anyone else about that. I think that that is obviously going too far and doesn't do justice to the way in which the formula is actually actually used.
>> I see. I I I guess my worry is just I I'm reluctant to have this suppose 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 I I don't necessarily think that's a justification for epistemic interpretation. And I think that's a justification for perhaps thinking more more carefully than we had 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 you're more sympathetic than I am to to the fact that we should indeed do that for classical mechanics. Yeah. Yeah. Fair enough.
>> A few months ago I was speaking to philosopher JB Manek. He's a philosopher of general relativity. And then he was telling me about the difference in style between himself and his adviser Malamment. 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, etc.?
>> Gosh. Um, well, I'm 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 um you know I think my my motivation has has always been been similar to to the physicists um and so the philosophy that I do even when it is quite on a quite philosophical topics I think for me is is motivated by wanting to understand what's going on with the physics and have better 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 Um, 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 to get a new view on those types of things. Um, and whereas yeah, I think I'm I'm more on the side of trying to use sort of the philosophical styles of thinking to clarify ideas about physics. So, I guess that's one one difference to some other types of ways of doing the subject.
David um my approach philosophy physics uh 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 and again. you see the widespread support for modifying quantum mechanics to solve the measurement problem. Um or for um minority approaches in quantum gravity or minority approaches in um in statistical mechanics or mathematical reformulations of of quantum field theory or statistical mechanics or etc etc etc etc. Um, I'm very keen on contemporary physics. Um, and very keen on trying to approach it with a much stronger sense of, you know, trying extremely hard to make sense of it as is. Um, partly just because I think that's ironically a somewhat niche activity in psychological. [laughter] Um I mean this has changed a little bit but I used to say that my my specialty was the narrow niche area of mainstream physics. Um 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 um one finds in in mainstream uh you know theoretical physics activity. Whereas a lot of the trajectory in philosophy of physics has either been has to go to the historical record um and look at how these these ideas developed historically or to look at extremely pure precise rigorous mathematical codifications of these things or to explore sort of dissident um programs. I think all of those are well modulate some of the distant programs. Most of those are extremely worthwhile things to do, but they're not the style of how I am interested in doing things. Um, are there any philosophical problems outside of physics that interest you both? David, we'll start with you.
>> Um, I mean, there's interest in interest, isn't there? I mean, I, you know, many things interest me. Um yeah, I'm interested in American politics and dinosaurs um and planetary science and um uh computer role playing games and so on. Um in terms of my professional research interests, um 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. And I am still I got involved in various questions in decision theory off the back of um uh the work I was doing the ever interpretation and I have some level of interest in those questions even outside the um many worlds context. I've done a little bit of published work in in those areas. Um, I'm sort of on the edge of having research interests in philosophy of mind, though again they sort of flow into my physics and structural realism interests.
>> Yeah, I mean somewhat similar. Uh, yes, there are many areas of philosophy that I am interested in, but I'm not competent to work in. Um, uh, I 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. So I've worked on the philosophy of lawood 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 selflocation because it ties into the ever interpretation and cosmology and I worked on um a few sort of philosophy of mind style things because I think it is relevant to concerns I have about time and the experience of time in physics.
So yeah I mean all of these things stem from the 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 and 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 to David?
>> Oh goodness. Um, I mean, I think there's a 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, um uh this is extremely difficult work but nonetheless it has to be done because the arguments for it are compelling.
>> Uh yeah I think as as David says his work takes seriously physics as it actually is and tries to understand and clarify issues in it and to have a better sense of what that what's going on in the physics and what that tells us about philosophy. I I I like that. I find that very helpful. I find very clarifying to read read that stuff and it certainly improves my own understanding of what's going on. Um, and I think I I personally am more willing to alter things, but I think nonetheless is really important if you're going to do that to really understand what's actually happening and why it's happening. I find Damon's work very helpful for that.
Well, I admire both of you and every time I speak with each of you, I realize how how little I know and how much farther I have to go and I feel like a fool, but I learn so much and I appreciate you both tremendously. Thank you.
>> Well, thank you Christopher.
>> Thanks.
>> Hi there, Kurt here. If you'd like more content from Theories of Everything and the very best listening experience, then be sure to check out my Substack at kurtjimong.org.
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