Rob Spekkens: Why the Quantum Wave Function Is Not Real

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Ultimately, I don't think any of these models are the right picture of reality. We need to try something completely different. Feynman famously said interference is the essence of quantum theory. My work showed that well, that's just not true. Feynman declared interference as the essence of quantum theory, with quote, absolutely no way to reproduce this classically. However, Robert Speckens of the Perimeter Institute proved him wrong.

In two thousand four, he found a theory of a classical world where your maximum knowledge is always in complete. What happens is out pops the no cloning theorem, teleportation, and even those interference effects that Feynman deemed impossible. We need to invest in people who have very different ideas of I'm Kurt J. Mungle. On this channel, Theories of Everything.

sidestep the recondite technicalities of a theory, as that's the whole point of this channel is to go research level deep. So allow me to explain some of the prerequisites For those who aren't physics professors. Speckens is trying to understand what exactly quantum mechanics is. What features of quantum mechanics are irreducibly quantum and not classical.

As you'll see in this episode, he argues it's Bell inequality violations. These are statistical correlations between distant measurements that exceed any classical bound. Most physicists conclude nature is non-local. Speckens disagrees. In 1716, Wydnes had a debate against Clark, saying basically, look, if you move every particle, say a few meters in absolute. It leaves every single observation unchanged, thus that displacement isn't real. Einstein actually wielded this principle twice.

Number one, eliminating the ether, and number two, identifying gravity with space-time curvature. This liveness principle is called the identity of indiscerning. Specken says that superluminal influences violate the same principle. Constructed to be undetectable. And if there is no empirical difference, then there's no ontological difference. Quite a controversial claim.

framework of hidden variable models. That's that apparatus that underlies psionic interpretations. Now psionic is just jargon meaning that the wave function is real, like Bohmian mechanics. It refers to an element of reality and this contrast With what's called psi epistemic, which is again just jargon, meaning that the wave function represents knowledge. Speckens wants to reject this dichotomy altogether. He compares our situation to Egyptian hieroglyphs.

Toward the end of this conversation, Specken says there's a similar category mistake we're making that Make when interpreting Egyptian hieroglyphs, a mistake we're continuing to make in interpreting our most fundamental theory, quantum mechanics. You challenge something quite sacred. So many popularizers of science love to talk about how quantum mechanics is mystical or magical and no one understands it, bro, and there's some assumptions

That physicists, many physicists have that these quantum quirks or bizarre aspects of quantum mechanics are unique quantum features like interference, superposition, entanglement. They're said to be non classical. Build for me the case that it is non-classical just so that people can get some context and of course explain any jargon along the way so that no one's lost and then we'll get to why you protest. Um yeah, I guess the

the the standard assessment of, you know, i is this classical or not. Um so first let me put out there that I I think it's an important question. So sometimes Uh it gets labeled as, oh, that's just semantics, you know, how we're going to use the language. But uh ultimately what's at issue here is, you know, what's what's the real innovation that quantum theory has brought relative to classical theories? And I think that's a really important foundational question.

Um, so to look at the phenomenology of quantum theory and say, okay, what in there is truly innovative is is important. Uh and traditionally I would say, you know, there there was a lot of phenomenology that kind of looked new relative to our classical theories. And so all of that was said, okay, well that that's sort of what's distinctive about quantum theory. Um and and that's fair enough.

But a a deeper question is, you know, w whether something really challenges the principles of classical theories. Right. So I might have some very particular classical theories in the past, you know, a theory of particles, a theory of fields. And then somebody might come along with something new, um, like say a theory of strings, and it it it's it's not overthrowing the principles of classical physics.

So the principles of classical physics might still hold. So it might be that dynamics is still described by a Lagrangian and all y the usual stuff. Still holds, it's just in a slightly different context. So the the more interesting question to some extent is, you know, which of the quantum phenomenology challenges the principles of classical physics? Um so yeah the standard list. Uh i is kind of things like you know, quantum interference, non-cummutativity of of measurements.

uh the fact that there's a no-cloning theorem, um a whole bunch of phenomenology of quantum information theory. So, you know, the fact that you cannot discriminate non-orthogonal states, uh, the fact that there's the

property called entanglement and entanglement has various features like monogamy. It can't be shared between more than and two parties. And there's there's just a long list of things that you would put say, you know, m most people think that this is quintessentially quantum phenomena. Okay. So uh yeah, the the the butt is that I I think most of that long list i is not actually violating the principles of classical physics.

Um so I have a a a different attitude and and what I really want to do is sort of find what more subtle features perhaps of that operational phenomenology actually challenges our classical principles. So m to do that I I think I need to take a step back and and kinda tell you about what what my principles are, where I'm coming from, because otherwise it none of it makes sense. So often in the discussion of foundations of quantum theory, a a lot of somebody's research program has to do with

you know, philosophical principles. And and so if we don't start there it it it won't make sense. So let let me take a detour.

And then we can get back to uh sort of which phenomenology which of the phenomenology of quantum theory is truly distinctive. Please. So so the the the the most important thing I think in discussing interpretations of quantum theory is, you know, w where do you stand on the dichotomy in the philosophy of science between empiricism and realism. Okay, so the empiricist point of view is that, you know, it's the job of a theory to describe what we observe in in in in experiments.

uh and it shouldn't really go beyond that, right? So that uh ultimately uh all of our descriptions should reduce to um Statements about what's happening in the experiment. Sorry, just a moment. Would the empiricists say that it shouldn't go beyond that or that it just doesn't happen to go beyond that necessarily?

I think a a a real empiricist would say that it is, you know, something people somebody like uh Moss would say it's fundamentally inappropriate to ask for more than just, you know, a a formalism that allows you to uh kind of compress the data, reproduce the phenomena. Um so yeah, there there is a sense that like i wanting something more like an explanation

or uh, you know, some some deeper understanding is is fundamentally inappropriate. And in particular, the sort of uh conceptual building block. ought to be, you know, facts that you can't really dispute, like uh facts about experiments. So the the the philosophical tradition I think goes back to people kind of wanting certainty.

So the fact that you can't really be mistaken about the things you've observed means that if you sort of define everything in terms of what you observe, you you will immunize yourself against being wrong because you know you Interesting. You're you're you're wedded to things that you just can't doubt. So that's kind of the the the background. I think that's where Uh, empiricism came from.

And realism, of course, is is looking i is granting that there could be concepts that are sort of not manifest, they're not defined in terms of sort of what we observe. Uh we still have to recover what we observe, but you know, the the deeper description might be in terms of concepts that are abstract. and we have a a kind of story about uh reality.

And so that th that's a big dichotomy in in views about quantum theory and the Copenhagen interpretation, you know, that that means many things to different people, but there there's a kind of common approach which says, look, let's just be good empiricists. Let's be uh in the quantum context we call it, you know, operationalist.

Uh let's just grant that quantum theory gives us an algorithm for predicting the outcomes of experiments when they're, you know, well described. And that's all uh a scientific theory should really do, so we should be happy. There's there's nothing more to to want. Whereas the realist is is looking for you know explanations of of those predictions, some something more. Um and so for from my perspective.

The uh I'm a realist, so I'm on the realist side of that debate. And and the thing that I see wrong with empiricism as a kind of fundamental philosophy of science. is that um first of all, it's sort of inappropriate to ask for certainty. So the kind of philosophical tradition that led to it was kind of based on a fallacy, I think, the idea that we could possibly kind of believe things and and not risk being wrong.

But but more importantly, I think i it's just an illusion that we can kinda go to the world, get data and and compare it to some theory because All observations are ultimately theory laden. So you can't really say what did the experiment yield, you know, what did it tell me, without bringing your realistic you know, uh presuppositions to bear. So if I'm looking at the, you know, positions of stars on the horizon at night.

through my telescope, I I'm bringing a theory of optics to bear. I'm I'm taking into account how those light rays are refracted by the atmosphere and my hypothesis and a whole bunch of theory is going into the interpretation of those observations. So I think it's an illusion that, you know, we we can kind of agree on what the observations are because our interpretations of those observations are always uh infected by our views on you know what's what's really going on.

Um so n nonetheless I think you know, to your point earlier that that there is a kind of weaker type of empiricism that's sort of more a methodological principle that says, Look, as long as we don't really understand what's going on, it's a good idea to try to uh

describe what we know uh in in kind of minimalist terms that most of us can agree with. So if we you know if we just talk about the statistics, the relative frequencies of outcomes in an experiment, then typically we can sort of agree on that despite our interpretational differences. So it's sort of a minimal statistical interpretation of what's going on that we can all agree with. And and it's a good way of freeing yourself from some presuppositions about what reality is like.

So Einstein famously used a kind of operationalist methodology when he started thinking about synchronization of clocks, for example, in in in developing special relativity. So he wasn't, you know, denying the importance of having a realist interpretation, but he was using uh kind of r restricting his set of concepts to operational ones in order to make progress, in order to sort of free himself of all the realist presuppositions of the theories that came before.

So I I think uh in quantum foundations, uh, you know, that that's something even the realists should be using uh that tool, you know, thinking about, okay, well, what is it that we have to explain? Answer it's the operational predictions of quantum theory.

That's sort of our goal of like what we're trying to reproduce with our our realist models. So, you know, Bell famously, in his theorem, connected sort of what you would see in an experiment to some, you know, deep principles about the nature of reality. Um anyway, so so for me, I I I kinda like a middle road. Uh so realism kind of informed

by operationalism. And uh there's a specific constraint that I want to impose on the realist interpretation. So so for me, this is the the most important principle in my research program. And I think most physicists

appeal to this principle, it sort of shows up and if you've, you know, done done a a degree in physics, you you're sort of familiar with the reasoning. But strangely we we don't usually talk about it as a principle. Um, so I call it the Leibnizian methodological principle because it goes back to Leibniz. Um so that so the way I want to articulate it, it's it's a version of of what Leibniz called the uh identity of indiscernibles.

So so the idea is y you know, I I like to say it's the ontological identity of empirical indiscernibles. So so what does that mean? Say I I I'm a theorist, I I have some realist theory of the world and and I imagine two scenarios. Um two two distinct physical scenarios. And I noticed that uh those two scenarios predict that what you would observe empirically are are exactly the same.

I i in that case, if if those are considered to be ontologically distinct, like they describe different states of affairs according to my theory. I should reject that theory because that would be a situation where you have no ability to empirically distinguish these two scenarios, and yet your theory says they're ontologically distinct.

Okay. So so Leibniz, for example, uh criticized Newton's approach with its positing of absolute space by saying, Look, if I took every particle in the universe and and and moved it over by five feet in absolute space, all observational data would be exactly the same and and therefore Uh, there isn't some real degree of freedom associated with the positions of particles in absolute space. There's only the relational degrees of freedom amongst the parts. That's the only thing that's real.

Right, so there he was appealing to the principle that you should eliminate any uh aspects of the ontology that don't have some empirical consequence. Um so I I I think that that principle shows up everywhere in physics and and we should use it. But the the main reason for thinking it's a good principle, I would say, is

the the use that Einstein uh put to it. Right. So if you if you look at Einstein's work, I would say there's strong evidence that this was the principle that really guided him. So Einstein of course you know about Leibniz's work and he was influenced by Mach and Poincaré, who were big fans of Leibniz. Um, but if if you look at like his nineteen oh five paper on special relativity, for example, uh so there he he's criticizing ether theory.

And he he says, Look, in the ether theory, you know, you can imagine, say, like a coil and a bar magnet and you can imagine the coil's at rest rest with respect to ether and the bar magnet is moving. And then here's a different scenario. It's the bar magnet that's at rest with respect to the ether and the coil that's moving. Um and so in in the in the case the first case you you get a current in the coil and and it's because, you know, the

time varying magnetic field that the electrons in the coil see uh has has a force associated with it. That you know time varying magnetic fields introduce an electric field, which means a force on the electrons, so you get a current. In the other picture, you you have um a Static magnetic field, but now the electrons in the coil are moving through that static magnetic field. So they feel a Lorentz force, and that generates a current.

But Einstein notes, the current's exactly the same in the in the two situations. There's there's no way to arrange this uh kind of experiment in in or any electromagnetic experiment that that responds to anything except the relative motion of the two. So to imagine that there's some significance, some ontological significance to the actual state of motion relative to the ether.

is to uh deny Leibniz's principle. So he says we have to come up with a way of thinking about things where it's o you know, we we have to enforce Leibniz's principle. It must be that it's only the relational motion that's important. And the formula of special relativity is Einstein's attempt to say, okay, here's how we can have a picture of reality where uh you know any two ontologically distinct scenarios correspond to empirically distinct observations.

He he does it again in in general relativity. If you look at what's the the his you know the famously his most beautiful idea, it's the equivalence principle. Following Galileo's thought experiment in a ship, but with with Einstein it's an elevator. You know, you you're on the one hand, you have an elevator that's at rest. In a gravitational field associated with some acceleration. On the other hand, you have the same elevator that's accelerating through free space.

absent of any gravitational fields with with a rate of acceleration that corresponds to the force of the gravitational field. And then famously you you see no differences. Any experiment you do looks exactly the same. So Einstein says, Well, any theory of gravity that says that accelerating through free space and being in a gravitational field are are distinct.

runs afoul of Leibniz's principle because there's no empirical difference. No experiment I can do can tell the difference between those two situations. So he says we have to have a picture of reality that does away with that distinction. And that's what general relativity achieves. It says, oh, the cur the local curvature of space time is exactly the same in those two circumstances, and and that's all that matters for the empirical phenomenon.

Um there's there's one other example where uh like the the very last step in nineteen fifteen before he got to general activity. Uh he he was trying to enforce general covariance. And um he had this thing called the whole argument where where there was sort of diffeomorphism you could do in just one region of space.

And it looked like his theory was sort of underdetermined that that there was you you know, the the predictions you would make would depend on whether this diffeomorphism had been applied or not. And uh so that he that kinda held him back. And then he suddenly realized, Oh, wait a second, the only thing that's empirically significant are are point coincidences, like when two particles uh arrive at the same point. And and and that uh is

uh invariant under diffeomorphisms. So he suddenly realized oh these diffeomorphisms I shouldn't think of them as ontologically distinct situations, right? Because there's no empirical consequence for these things.

So then when he realized that and he said, okay, the only ontologically significant thing is sort of what's what's invariant under diffeomorphisms, uh, because of Leibniz, you know, he wanted to enforce that, he he was able to see how to impose general covariance, and then he had general relativity.

So for me, uh this principle has been extremely effective, you know, in in in the development of special and general relativity. Uh I think it's very plausible. I think it's it's a great principle, you know, for us to be using and I think we we use it all the time. So to some extent Uh it's it's the thing that I hold on to and I wanna ask the question, you know, what interpretational quantity are you led to if the principle on which you don't compromise is this Leibnitsian principle?

Um you know Einstein of course could have made very different decisions at various points when he, you know, encountered difficulties. He could have given up on Leibniz and tried something else. So I see a lot of his success as being due to sort of never compromising on that principle. And so that's kind of uh dictates my research program as well, that, you know, what do we get to if we don't want to compromise on on Leibniz? Now are there counterexamples to this Leibniz principle?

Um well it's it's it's not a principle that you can go and and test. It's rather a principle that constrains your kind of

realist theories that you ought to construct for a given set of data. Sorry, I'll give you an example. Yep. My understanding is that French and Redhead in nineteen eighty eight or so They showed if you have two bosons in a symmetric state, maybe you've seen this already, that they can share all their properties that neither has some definite position and and they're all symmetric under some relational properties.

Yet we still would call it a a member of a two-particle state in Fox space, and not just one. When I'm wrestling with a guest's argument about, say, the hard problem of consciousness or quantum foundations, I refuse to let even a scintilla of confusion remain unexamined. Claude is my thinking. Actually they just released something major, which is Claude Opus four point six, a state of the art model. Claude is the AI for minds that don't stop at good.

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Claude.ai slash theories of everything. That's Claude.ai slash theories of everything. And check out Claude Pro, which includes access to all of the features mentioned in today's episode. Yeah, I think y you're referring to so the place in physics where people usually talk about Leibniz's principle of the identity of indiscernibles is is when we're talking about particle statistics, bosons, rummions, that sort of thing.

So so I'm you know, the first thing to emphasize is I I'm using it in a very different way as uh a kind of methodological principle for like what kinds of realist theories should you entertain. Um so it's it's Yeah, I I don't really have a a strong view on what should we think about, you know, the the identity of two particles. Um Yeah, I I think that's a that's kind of a a a different direction in which to take Leibniz's ideas. So I'm I'm more

I I was led to it because uh when you study non-contextuality in quantum theory, you can uh ultimately justify it via Leibniz's principle. So that's sort of w one of the reasons it it appealed to me, that is sort of the principle behind non-contextuality. But but ultimately it's it is a principle, right? So I think we you know we we can argue about whether it's something we should impose or not. It's not something we can really test.

Um I I brought up Einstein in the historical success of the principle because I think in in physics it is wise to sort of look at the history and say what kinds of principles have tended to lead to progress? You know, th things like, you know, energy conservation is a great principle where, you know, uh you you ought not to give it up if you can avoid it. That that generally

progress comes by being very conservative at the principle level. Most of the principles that have proven to be effective in past physical theories survive in the new theory. Uh and it's usually something more subtle that changes, something about the background assumptions. N not these principles. So um that's the spirit in which I'm using it. So it was quantum contextuality that led you to think about this modified Leibniz principle. Yeah, I I think uh I mean I I I knew

I guess something about the the literature on kind of relationalism in the study of of gravity. And I was sympathetic to the the relationalist program, which really sort of comes from Leibniz ultimately, Leibniz and Huygens. Um so I I think I was already uh open to that. And and maybe it was my sort of

knowledge of, you know, like Le Leibniz articulates this pretty well in the in the correspondence with Clark, which is basically Newton's spok spokesperson. So so I knew about that and and then I I guess when I was studying non contextuality, I sort of saw that same pattern and so I made the connection and

And uh yeah, that's that so now I would say it's it's liveness I'm really committed to and it's the thing that underwrites my belief in things like non contextuality and locality and other principles.

So anyways, that's that's principle number one. And then maybe the the second thing to say about my views on realism is Realism is kind of nebulous. Like what counts as a realist interpretation uh of quantum theory? Uh and so where I've landed uh i is that there's there's there's one aspect of realism that I think is is important. So this is the kind of realism I want for quantum theory, which is that correlations should have causal explanations.

Um so you know, i i if somebody says, Hey, there's this experiment and and there's some correlations, but I don't feel any need to answer the question of whether, you know, these two variables are correlated because one influences the other or because there's a common cause if they just say, no, I I don't care about why, uh I just say they're correlated.

And for me that's a kind of anti realism. Because, you know, in in in the context so I I spent a lot of time thinking about uh modeling things causally and there's this field of research called causal inference, which people in statistics and and uh computer science study. And you know, if if I have uh data that tells me, you know, ta taking some drug is correlated with recovering from some condition.

That doesn't mean that the drug was effective, right? Because there are other ways of explaining that correlation. So it could be that you know, uh gender influences both whether you take the drug and whether you recover. So men are more likely to recover from this condition than women are. But men are also more more likely to go out and get this particular treatment or or medication. So in that case, it might be that there there's no causal connection at all.

And when you learn that somebody took the drug, you do update your probability about whether they recovered. You say it's more likely that they recovered, but not because of the causal effectiveness of the drug. Rather you say, Well, because they took the drug, I I I infer that they're more likely to have been a man, and because they're a man, they're more likely to have recovered, and so that's the explanation of the correlation.

So in that context, i you know, you wouldn't want to say it doesn't matter what the causal explanation is, because obviously, you know, whether it's a common cause or cause effect entirely determines whether you would wanna take this drug or not. That's really the question you wanna know.

um it you know, is the drug causally effective? So I think across science, what we're often looking for is causal explanations of the correlations we see. And I think it's no different in in quantum theory that to understand what's really going on, we need a causal account.

And so I'm willing to give up on certain aspects of classical realism or the the conventional framework for thinking about realism, uh, but at the end of the day, uh I want to insist that it has to provide causal explanations of correlations. Um so that's kind of where I'm I'm coming from. Um and and ultimately uh The, the...

the these two things are intention with one another. So there's, you know, commitment to Leibniz's principle and wanting causal explanations of correlations. Uh there's there's a a strong sense in which they their intention, if you wanna understand quantum theory, And that's because we have these no go theorems that basically say, uh, if you believe the quantum predictions are correct,

and you subscribe to the conventional framework for describing realist theories. So so I often call this the ontological models framework. It's it's sort of a a a picture of reality where you say, Okay, variables are just things that take a set of possible values.

uh you know, dy dynamics is basically functions, my ignorance of the values of things. Well that's described by Bayesian probability theory. And I can use that to calculate, you know, what sorts of relative frequencies we should see in experiments. So this is, you know, i exactly the kind of thing that used to be called the hidden variable model that Bell used and Kosh and Specker used in proving their no go theorem.

Um and what you can show is that this commitment to the Leibnizion principle actually it's it's strong enough to get you to Bell's notion of local causality, to get you to uh Koshen and Specker's notion of non contextuality, which which we may talk about later. And so really all these no-go theorems can be thought of as if you subscribe to the conventional framework for realism and you believe in this uh Leibnitsian principle, you're gonna get a contradiction with the quantum prediction.

Um so y yeah, you know, what are you gonna do in the face of that? Uh So I would say a a common response is to kind of give up on Leibniz, right? So when people say, let's imagine that there are superluminal influences that explain Bell inequality violations in a Bell experiment.

or let's imagine that there's some hardwired context dependence in my hidden variable model. They're they're giving up on Leibniz. Okay. So I and and I'm un unwilling to do that. So so what I would rather say is Well, the th there's aspects of the conventional framework for realism that we can relax, that we don't have to do it the conventional way.

So if we all we're demanding is that we have causal explanations and correlations, we can have certain kinds of realism. They're just not gonna meet the you know, they're not gonna be described by the classical framework. And so that's sort of My uh uh attempt to to make sense of quantum theory is that, you know, roughly speaking, the innovation is gonna come in.

in how we think about causation, how we think about inference. Right. So those are those are gonna be two really important elements that that we need to unscramble. Like what's what's about reality and what's about our knowledge of reality. So the you know the drug trial example was a really, you know, nice example of that, which is like,

You want to distinguish whether, you know, learning about whether somebody takes the drug merely informs you about whether they're gonna recover. You know, this that's just or whether, you know, the taking of the drug actually caused them to recover.

So a lot of what I do in quantum mechanics is trying to, you know, what Jane's called the omelet of epistemology and ontology in in quantum theory. So I want to know what's about causation, what's about inference. And so roughly speaking, you know, for me You know, the the there's going to be quantum notions of causation and inference, and they are going to be innovations relative to the classical notions of causation and inference.

Much like, you know, if you think about the notions of space and time, relativistic notions of space and time are innovations on the pre relativistic note. So th they're still recognizably about space and time, but there's certain aspects of the conventional notions that we have to give up. And so it's similar here that there's certain aspects of the notions of causation and inference that we're we're gonna have to give up when we we go to quantum theory.

So that's the the the background uh the the the research program. Uh so maybe now's a good time to come back to your question about, you know, why why the usual list of surprising quantum phenomena I I don't consider to be surprising. Um And and the reason is that if you you you can

write down some some models that are classical, you know, they they obey classical principles. And the only real innovation relative to classical theories is that these models say there's a restriction on how much you know. Uh so there's sort of like if if you ask how what's the maximum amount of knowledge you can have about some physical state, i the answer is it's still incomplete knowledge. Um

So so these so okay, so so for me, this came out of thinking about the nature of the quantum state that uh I'd I'd heard some you know a long time ago. Uh I I heard about arguments in favor of the epistemic nature of quantum states. So that means quantum states are representing knowledge. And I was sort of exploring how okay, how much of the phenomenology of quantum theory can be explained if that's the view we take. And it became apparent that you you can explain a lot. Um

So so you know, what's what's a good example? Um The the like okay in quantum information theory, we have this property that if you give me two non-orthogonal quantum states. then there's no measurement that'll allow you to determine with certainty which one it was.

So if they're orthogonal, then there's a measurement that'll distinguish them. But if they're not orthogonal, so in the in the block sphere picture, they're they're not antipodal, then there's no quantum theory predicts there's no measurement that'll allow you to discriminate them with certainty.

If you take the view that the quantum state just describes all the properties of the physical system, so that that's the view that is sort of a complete description of the reality of that system, then it's kind of surprising. It sort of suggests that, oh, there's a limit to, you know, the kinds of measurements we can do.

If you take the view that the right way thing about quantum state is that it's epistemic, then then as a classical analog of a quantum tate state is is not a point in a physical state space, but rather probability distribution.

over the physical state space. So so one of these quantum states might be, you know, a distribution like this. Uh another could be a distribution like this. And being non-orthogonal corresponds to those distributions overlapping somewhere. Right. So if you say, okay, let let's think about state discrimination in quantum theory from that perspective. So so now the perspective is

You know, I give you a a system and I tell you, look, it was uh either sampled from this probability distribution over here, or it was sampled from this distribution over here. Your job is to figure out which it was. Now these distributions overlap. So some of the time the physical state I'm sending you is in the region of overlap. And in that case, obviously you're not going to be able to tell which distribution it was sampled from.

I if you figure out that it's in a region that is only consistent with distribution one, you can say, Oh, it was it was definitely distribution one or similar with distribution two, but if it's in the region of overlap, you'll have to say, I don't know. And and

Uh and so you expect if if quantum states are interpreted this way that you ought not to be able to tell which distribution was assembled from, and indeed that's what quantum Okay, let me spell this out with a different analogy for some of the regular viewers of of this channel. So men and women's heights are normally distributed, but men tend to be slightly taller.

So if you were to look at people who are seven foot and above and you knew that seven foot and above were only men, and I told you, okay, I'm just going to tell you the height. I'm not going to tell you the the sex of the person.

I want you from the height to infer the sex. Right. Seven point one foot. What what is the sex of this person? You say, Well, it's a male because only seven foot and above are male. Right. And then I say, Well, five foot five. Then you're like, Well, I I have no clue. I mean

forty percent of them could be w or s sixty percent could be women and forty percent could be men. You could say something like that. So you have in the overlap you have some uncertainty. And you could also have a in this case, you could have It's not just you're completely uncertain, you can have a model for it, like sixty percent women, forty percent men. Yeah. Yeah, that it the I I like that example. It

I may maybe somebody could say, Look, there's always a non zero probability that there could be a woman who's who's seven foot tall. Of course, yes. Sorry. But let me modify that now. It's not a normal distribution that in w what I just said. Otherwise there would always be a non zero probability, right? But yeah, it's it's it's like the I mean if I have one deck of cards and I say, look, there's there's only hearts

and and spades here. And here there's only hearts and and clubs, right? And and now I draw a card and I give you one and I say, well do you know which deck it was drawn from? Well if it's a spade or a club, you're like, yeah, I know exactly which deck it's from. But if it's a heart You're like, well, that's consistent with both, so I can't tell them apart.

So like that example makes it clear that okay, there's zero probability of drawing a spade from here, and there's zero probability of drawing a club from here. So when I get those results, I'm certain. Uh and and that's kind of how these these models work. that uh you you have uh classical probability distributions that assign zero probability to certain outside.

So that's one example of like the phenomenology of quantum theory that you cannot discriminate non-orthogonal states arises in models where the pure quantum states get represented as probability distributions. um that are not point distributions, right? So they have some spread, right? They're consistent with many physical possibilities and and they tend to overlap. So you can explain uh you know just that that simple idea

gives you intuitive accounts of a lot of phenomena. So if you look at the no cloning theorem, for example, um, and you ask, well, let let's phrase it in terms of probability distribution. So what is no cloning? It says, look, if if you give me a system and you tell me it was, you know, either quantum quantum state Psi one or psi two and they're they're not orthogonal.

And now you have another system and and you're trying to sort of copy the quantum state onto the other system. So you're if it was psi one, you want to be psi one tensor psi one at the end, right? Two copies of psi one. If it was psi two, you want to Psy twos, tensor side two at the end, two copies of side two. Um so it's the same. If if I say, well, the way you ought to understand that is somebody's given you a sample from one of two distributions and they overlap.

Now please make it so that at the end of the day, your knowledge, your your your state of knowledge about the pair of systems is just, you know, what whatever uh the probability distribution pertained initially, it now pertains to both. Um but there there's there's no way of of doing that. If if I copied the physical state, I would generate correlations between the two, and that's not represented by a product distribution.

Um and there's this, you know, there's always these ontic states that are in the uh the the overlap of both. So you cannot figure out what the distribution was by, you know, measuring it. So you can prove that there's there's no way of doing this with classical probability distributions. It's just a very natural fact about distribution. And and so you you start to realize that when you look at what happens with quantum states

Uh, so you know, they they can't be cloned, they can't be discriminated. You can teleport them in a certain protocol, you can steer them in the Einstein-Podolsky-Rosen steering experiment. You can sort of Uh look at all these things and say, is it the case that that sort of stuff happens with probability distributions as well?

And the answer comes back yes. Uh and in particular, if you have a model of a classical world where there's a fundamental limit on how much you can know, so so your probability distributions are are never too narrow, they're they're never down to zero entropy. uh then you could do a really good job of of reproducing the qualitative aspects of this long list uh of things that occur in quantum theory.

Um so that's that's what my uh toy theory from from uh 2004 does. It sort of shows what is all the phenomenology you can reproduce. Um and that that

What led me to say, well, these are the sort of standard list is not the thing we should really be focused on if we want to know where the real innovation of quantum theory lies. So so it's not that I think these models are empirical competitors to quantum theory. They're they're not.

because they're things that they can't reproduce, right? So Bell inequality violations provably, like I mentioned earlier, there's no go theorems that say if you subscribe to the conventional views on on how to think about realism, which these toy models do. And you believe in the Sleibnitzian principle, which means that you have to have locality and non contextuality. And the toy models have all that.

Then there's some phenomenology quantum theory you will not reproduce. And Bell inequality violations are one of them. So this is a particular kind of set of correlations you see in a Bell experiment. So from the very beginning it was sort of clear that these uh models aren't competitors to quantum theory. Uh but we we study them as as foils. So so I I I like to talk about the methodology of foil theory. So

A foil is is something that you study because it contrasts with the thing you care about. Right. So we care about quantum theory. We want to understand it. But part of understanding a theory is knowing what are the other possible ways the world could have been. Um so so my toy theory is is, you know, you could think of it as a way the world might have been.

uh it's different from quantum theory. Okay. But by studying it I I I realize you know what is truly distinctive about quantum theory. So this toy theory is very conservative in terms of like the it imagines the classical reality and and a

a relatively modest change to our classical theories, which is we can't know everything, and yet it reproduces a whole bunch of the phenomenology. So so what is it that's truly distinct about quantum theory? I can learn a lot about that by studying these kinds of foils. Um so so ultimately, you know, here I I study these these foils as a way of saying, well, what are the phenomena, the operational phenomenology of quantum theory that really do resist?

the kind of conventional framework for realism together with the Leibniz in principle. They cannot be interpreted in that way. Because that's your best guide to how to make progress in a research program where you're you're trying to hold onto Leibniz and innovate on the framework for realism. Distinguish for me toy models versus toy theory.

Well, I think the the way they get used is is often synonymous, but personally I I like to use the word model when when I'm modeling something. So like a a model of quantum theory might be like a a particular account of how you reproduce the quantum predictions. Whereas a a theory could be, you know, an alternative to quantum theory that makes different predictions.

So this toy theory I was talking about is an example of that. It it makes different predictions from quantum theory, so uh in in certain circumstances. And uh and so I I don't think of it as a model of quantum theory. Uh it's rather a different theory. And, you know, the the word toy theory was was meant to just kind of a list you know, make it clear that this is really being

viewed, you know, not seriously as an empirical competitor to quantum theory, but as a foil, as as something that will help us learn in particular what what principles might underlie quantum theory, right? So it is for ex so there's this research program Um Where where you you imagine a landscape of possible ways the world could have been. So so people use different formalisms for talking about points in this landscape.

One one is called the the framework of generalized probabilistic theories. And essentially it's sort of like saying, look, the operational stuff that we can all agree on, like what are the statistics we see in experiments, that's what can vary as I vary over this landscape. Um and uh

You know, what one of the things that people in quantum foundations like to do is try to pick out some axioms that might find locate quantum theory in that landscape. It's the only theory that you know abides by these particular principles.

Um so like, you know, you might have said, oh, well, the only theory that allows you to violate Bell inequalities while still not signaling superluminally is quantum theory, right? So that would be a a conjecture. Well, that conjecture is false because you can find a FOIL theory, another point on the landscape. uh where you violate bell inequalities more than quantum theory does, but you still can't send any signals.

Okay, so this was famously proven by Pepescu and Rorlich. And you can develop those ideas into a whole theory, which is sometimes called box world. So that's like a point in the landscape, different from quantum theory. Right. And so, you know, if somebody Thought that those principles could get you to quantum theory, you know, box world shows no. No, there's there's alternatives to quantum theory that satisfy those principles. So you haven't captured the true essence of quantum theory yet.

Similarly with these toy theories that, you know, if you thought that, I don't know, no cloning and teleportation and some other list of phenomena was enough to get you to quantum theory.

These toy theories show you no, like that that phenomenology is reproduced in other theories distinct from quantum theory. So it cannot pick out what's truly distinctive uh about quantum theory. When I'm wrestling with a guest's argument about, say, the hard problem of consciousness or quantum foundations, I refuse to let even a scintilla of confusion remain unexamined. Claude is my thinking partner here.

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Ready to tackle larger problems? Sign up for Claude today and get 50% off Claude Pro when you use my link Claude.ai slash theories of everything all one word. Okay, so if I'm understanding you correctly, there are various features of quantum theory that I mentioned earlier, such as s such as superposition entanglement and interference, but there are others like bell correlations and maybe

Four more here, or twenty more, whatever. Yep. The point is that you then wonder okay, of these features that we think of as being non classical. Can we develop a classical theory that triggers some of them? And you have found, yes, for interference, entanglement, and Superposition. Yes, the answer is yes. Right. But you have not been able to find it for bell correlations and and so forth and so forth.

And then I heard you say or I thought I heard you say that we have shown that you can never recover s some of these fingers here that I have on screen, classically. But it sounds to me like the latter part of what you're saying is that we just have not found a point that can that can actually recover them. So what one is that there doesn't exist something versus that we have not found it. So it's it's the first one that it doesn't exist. So the um

We're we're when we're we're showing that something can't be explained under some set of principles. We're we're proving a no go theorem. Uh so so you might say take take the conventional framework for realism, which I like to call the ontological models framework. It's it's about, you know, hidden variables, Bayesian probability theory, that sort of thing.

Assume something called local causality, which is Bell's way of talking about things being local. I I like to think of it in a causal model. There's only a common cause between the two wings of the experiment. Um relative to those assumptions, you can prove that the only correlations you get will satisfy certain inequalities. The quantum formalism violates those inequalities, so you can say, right, I now have a contradiction from those assumptions.

And so I know that, you know, one one of my assumptions has to be wrong. Uh so it, you know, might be the local causality. And a lot of people say that was the wrong assumption, that actually there are superluminal influences going on. You know, that that's what's innovative about quantum theory. But I'd rather say no, it's still a common cause explanation, but it's this conventional framework for thinking about realism, for thinking about causation and inference. That's what's uh gonna change.

Um, but it's it's not that we uh don't know whether there's some model that can reproduce this. Uh it it's it's rather we've proven a theorem, right? So it there there are certainly models that can uh abide by the conventional framework for realism. and violate bell inequalities. It's just that those models run afoul of Leibniz's principle. And so that's why I I don't care about them. Right. So th so let's say I I try to

uh build a model that really has superluminal influences in it. So that that's how I explain what's going on in a Bell experiment. I say, okay, the the set, you know, we have Alice and Bob. Uh that there's a pair of particles that gets prepared at a source and and each one gets a particle. And then uh Alice has a setting variable that determines what she's measuring and an outcome, similarly for Bob.

So if I allow Bob's outcome to depend not only on, you know, the the physical state of the particle he got and the setting variable, but Alice's setting variable a well as well. Right. So that would have to be a superluminal influence because they can do these measurements at space like separation. So that influence is gonna have to go faster than the speed of light. If if I allow that, then yes, I can explain the the Bell inequality by

Okay, but why should we be dissatisfied with this? Well, you know the the usual story you hear is Uh you know, people who who trust relativity say, Well, it it just it just feels like relativity is not merely saying that signals can't fast uh travel fast and speed of light. It's saying that influences can't travel fast in the speed of light. So so you might just say, well

it it seems intention with relativity to say that there could be an influence that travels fast and the speed of light. But but you can do better than that and you can just say, look, if you believe in Leibniz's principle, then this is clearly not Leibnizan because

Uh, you know, imagine, you know, uh Einstein's elevator thought experiment. I I go to Bob's lab and I do every experiment you can imagine looking to get some information about w what Alice's setting was. And lo and behold, no matter what I do. According to relativity theory, because they're space like separated if I And and there's no signaling. Nothing I do in Bob's eye can teach me anything about what's happening inside.

uh uh Alice's lab. So so yeah, what I do in Bob's lab can't teach me about Alice's lab. So at the empirical observed phenomena scale, there's no evidence of of uh any influence, no, no signals for any experiment that you could do. But to imagine that there's nonetheless an influence, well, that's violating liveness. That's saying that there's an ontological difference, but you cannot see it, right? No experiment you do will see that difference.

So that's the sense in which Bell's notion of local causality follows from Leibniz's principle. Um and that's why, you know, I I would uh not want to give that up. Uh So it's it's the conjunction of Leibniz's principle, together with this conventional framework for realism, that leads you to contradiction. And and so that's the sense in which I could say, look, there's a certain kind of

You know, that there's no toy theory like construction, you know, that it'll satisfy Leimnets, it'll it'll be in this conventional framework that's going to reproduce the Bell inequality violations. That that's clear.

Now m more generally, that's you know my take, but more generally what I'd say is, you know, in quantum foundations, it is good to follow this methodology of no go theorems, right? So like if if we just argue about, oh, I got this model, I want to call it classical, I think it's nice. Um if I look at like what Feynman wrote. about things like interference. So, you know, Feynman famously said interference is the essence of of quantum theory.

Uh so some of my work showed that well, that's just not true. Like I can reproduce he's you know, Feynman said there's there's absolutely no way to reproduce this classically. I I say that's not true. I I can have a classical model. reproduces the phenomenology. And the problem with what Feynman said was that he didn't prove a no-go theorem. He he didn't say, this is what I mean by a classical explanation, you know, principles.

Here are the predictions of quantum theory, here's the contradiction. If Feynman had done that, then then all we could do in response is maybe dispute those assumptions. We might say, Well, that's not how I think about classicality. So so I think we escaped the contradiction by giving up assumption one.

Um and and that's what's so nice about the methodology of no go theorems in quantum foundations. Like we we all researchers and quantum foundations tend to disagree about what the fundamental principles are. But what's great about a no go theorem is that if you agree that it's a logically valid argument, you have to say which of the assumptions or or many of the assumptions that you disagree with

And why. And so it focuses your as it focuses your attention on the principles and how should we articulate these principles and are they reasonable or not? Um so yeah, that that's the sense in which somebody you know could always say, all right, you've proven a no-go theorem, but here's why I don't think um it's very significant, because I think the assumptions you put into it aren't natural assumptions.

Uh so so most people agree that, you know, Bell's assumptions and Coschen Specker were were pretty natural in some sense. And so those no go theorems are significant, but you know, other no go theorems might not be because they're built on assumptions that are just not natural that nobody really thinks are are plausible. Can you explain to a mathematician what is special relativity such that so the mathematician may say something like it's invariant under SO one three?

Some theory that's invariant under that. Now of course there may be more conditions, but either way. Once you've specified what special relativity is, what the heck is the difference between something being non locally influenced and then And that some people will say, well, special relativity allows non-local influences. It just doesn't allow you to communicate non-locally or superluminally. Like where where is that said in special relativity? Like explain to a mathematician.

Yeah, um I mean if i if you take, say, Maxwell's theory of electrodynamics Uh clear that you have neither any superluminal signals nor any superluminal influences. Right. So it has the Lorentzian symmetry, you know, it it it has the right symmetry to be consistent with special relativity. And and and it has neither of these these features.

It's it's only when we start asking about quantum theory, characterized totally operationally, where we're like, okay, well we just have these statistics of outcomes of experiments. And, you know, what what Bell asked was, is there any picture of reality? You know, could be Maxwell's theory, it could be Fields, it could be anything. I I don't care what the underlying theory is. Is there any theory?

That respects the following aspect of relativity, namely, that influences do not propagate fast and speed of light. So um he he was assuming the the structure of space time that's in relativity. So I can find two regions that are space like separated, meaning that, you know, there's there's no influences between them.

Uh that that's what you're stipulating that you know, if yes, if you imagine that all influences have to propagate within light cones, then I can find two regions that are outside each other's light cone. I can use that for the Bell experiment, right? The the settings on Alice's side are outside the future light, uh outside the backward light cone of Bob's outcome. And therefore, if I believe that influencers are subluminal, there there can be no influences there.

So that's i it's it's not the you know, f uh the full apparatus of special relativity that's appealed to in Bell's argument. but rather just this aspect that says there is a fundamental speed limit, and therefore there can be two regions of space-time that are not ordered causally. Okay, what's meant by influence? And is influence the same as causal influence?

Yeah, I've been using influence to mean causal influence. Um Is there such a thing as a non causal influence? Like like help me understand what influence means. A good word to talk about a cause-effect relation. So, you know, variable X influences variable Y, that means that you know uh X is a cause of Y. Um so the the in so far in our discussion I've been trying to distinguish influence from signaling. Right. So you could say, um yeah, yeah, maybe here's the the best example to see how the

They really come apart. They're not the same idea. Uh it's it's from cryptography. Um so okay, let me let me get there in a moment. Uh so so you know what what's the way in the field of causal inference that causal influences are mathematically modeled. You can think of it as just its functional dependence. Um so so the key is unscrambling this omelet of uh inference and and and and influence.

And so if I think of like projectile motion, right, I can write down a formula that relates, say, the maximum height of the cannonball, the initial velocity of the cannonball when it came out of the cannon, and the angle of the cannon. So I can write down a formula that relates

And if you give me any two of those variables, I can infer what the value of the third is, right? So I can say, oh, you give me the maximum height and the the angle, I'll figure out what the velocity must have been. You know, so that's undergraduate physics. Allows me to write that equation and solve for any of the three variables.

But the only one of those equations that really dis describes a cause-effect relation is the one for the maximum height in terms of the velocity and the angle, right? Because if I modify the initial velocity or modify the angle, that will impact what the maximum height is.

But if you know the cannon has been shot and I go and I move it up a bit, well, that's not gonna influence the cannon. So so we all kind of intuitively understand the asymmetry of causation, right? Uh the inference is symmetric. I can make inferences between lots of different variables. But influence, causal influence, is is asymmetric. Uh another good example would be um like uh if you think about the the rooster crowing and the sun rising, there's a correlation there.

So you might say, okay, well what's what's the causal relation? Uh well if I make the rooster crow in in the middle of the night, it it doesn't cause the sun to rise. But if I move the rooster eastward on the earth so that the sun rises later, it crows later, right? So I I can figure out what the causal influences are based on interventions. Um but also our theories of physics teach us something about, you know, what what are the causal relations, like in the cannonball example.

So eff effectively causal relations are described by functional dependencies, but they're very specific functional dependences, right? Because inferences can also be functional dependences. There are functional dependences where we we imagine that if we know say y is a function of x, and it were I to vary x, y would vary. That's effectively a a causal influence. Um okay, so now uh why is that distinct from signaling?

So here's the the example. It's from cryptography. So there's something called the one time pad in cryptography, right? Sometimes called the Vernum cipher. So say uh I have some plain text, some sequence of bits, zeros and ones, and I want to communicate them to you, and we share a key, so some secret string of zeros and ones. And that key has been, you know, generated at random from all possible sequences.

Then I can add the key to the plaintext, send it out. That's the ciphertext. When I add them up, mod two, send it out in the open, you get it, you add the key, you get the plain text. So in the simplest example, I'm saying sending you one bit, that's the plain text. The key is just one bit. I add them together, mod two, that's the ciphertext. Now the way this is designed is that the the key is sampled at random from a uniform distribution. It's either zero or one with equal probability.

And therefore the ciphertext, if you say, well, what do you learn about the plaintext? Well if if the key was zero, then the ciphertext is equal to the plain text, and if the key is one, it's just the flip of the plaintext. And the eavesdropper gets the the the the ciphertext and says My estimate of the probability of the plaintext being zero is a half, and the probability that it's one is a half. And so the eavesdropper has exactly zero information.

as if you were guessing, right? So they information theoretically, they learn nothing about the plain text. That's the whole purpose of the protocol. Now, if if you ask about signals, right? So say I vary the plaintext, and I say, well, what's the distribution over ciphertext? that results from that variation. that the ciphertext is always uniformly distributed, zero or one, because the key is uniformly distributed zero or one. So it's equally likely to be zero, one.

And if I am sending a different message, it's also just gonna be the uniform distribution over 011. So that's an instance where there's there's no signal from the plain text to the ciphertext. Okay. Changing the plaintext is not gonna change the distribution of the ciphertext at all. That's no signaling, right? There's no change in the distribution. But of course, there's a causal influence.

Because if there were no causal influence, there'd be no way that the recipient could decode the message. There has to be a causal influence. So the the the Vernum cipher shows you that there can be causal influences. where you kind of wash out the influence. You don't get to see it because you've sort of added noise. Um so signaling is stronger than influence. It's like, oh, there's an influence. And furthermore, it's not being washed out by a noise source.

Um, but I can have influence with no signaling. So that's the sense in which they're they're different concepts. And so in Bell's theorem, when when you look at models like Bohmian mechanics and other invariable models,

that manage to reproduce Bell Inequality violations using superluminal influences, they use exactly this trick that they they sort of wash out. They say, we have influences, but we can't use them to signal. Why? Because we're sort of washing out the influence by adding local noise. So it's the Vernum cipher trick happening in all these these hidden variable models. That's how you uh m make it consistent that there's no signaling and yet influence.

Uh, and so that works fine, unless of course you want to stick to Leibniz's principle, in which case the fact that nothing you ever do can see that signal uh means that you should not be assuming that there's an influence there. Perfect. Okay. Okay. I want to talk about causation. Okay. Let's go to how physics is ordinarily said. I'm not talking about the equations on the board. Most of the time when people are talking about physics, talking, they'll

speak about physics in terms of causal accounts, I'll throw the ball and that will cause you to d fall down to the ground when it hits your face or or what have you. Okay. Then when they're drawing equations on the board, they're just equations with evolution. And then you could always ask the professor.

Where is causation here? Like what variable is causal? What what's going on? And then they may say, well, ultimately in physics, fundamentally, there is no causation, that causation is a story. And even in this this other account of a projectile being thrown or the rooster crowing Emily Adlam, who I spoke to a few months ago and I'll place the link on screen, my recollection of or my understanding of her theory is that

There is no causation, and you can also tell a story where the end is somehow constraining the beginning part. So these variables that you mentioned with the maximum height. And the angle and so forth, you could just give the maximum height, and you don't have to talk about causal stories. Then when I heard you speak about causation, you said interventions. Now interventions to me imply agents, and so if one was to give a fundamental account of causation,

it would seem to me that you're placing an agent or an intervention fundamentally in physics. And I j I just don't see that there. So I need help to understand. Yeah there there's certainly so so I don't subscribe to the view that we we need agents and the notion of intervention to define causation. There are some researchers who certainly do push for that view of of causation that it's sort of defined in terms of intervention.

I would rather say that a a good way to get a handle, a good way to get evidence about what the true causal structure is, is with intervention. So if if I can afford to do a randomized drug trial, that's a kind of intervention where I um give somebody a drug or a placebo, not based on their preferences.

but just based on the outcome of a coin flip. And then I can be sure that when I see a correlation with recovery, it's by virtue of the causal effectiveness of the drug. Because there was no common cause to the outcome of the coin flip and and their recovery. Unlike an example where

you know, uh health awareness or something might influence somebody's decision to take the treatment and influence their likelihood of recovering. Right. Um so interventions are a great way of getting a hold of what's going on causally, but I don't think it's part of the definition of causation. I would say rather we have to reach

for uh just, you know, the the notion of counterfactuals that um, you know, the the when I think about classical physics and I say, well, what does it mean to have a the law of projectile motion? In what sense is it a law? It's not just a dis you know, an account of every, you know, cannonball flight that ever was in history. No. It's a law in the sense that it allows you to reason about counterfactuals, to make inferences. Had the velocity of the cannonball been different?

This is what the maximum height would have been. You know, it it allows us to make all those inferences, right? So it it basically stipulates a functional dependence of some effect variable on the variables that are the causes of that effect. Um so yeah, I think uh generally we we we we don't kind of formalize it. We that's all kind of in the the words that go along with things. So if I'm doing a problem on uh, you know projectiles of cannonballs and

And and I get asked, you know, what what happened if you went and you modified wh where the cannonball went by grabbing hold of it and moving it, you know, would would that impact the the the cannon? And you know, no, of course not. Like we we we know enough to understand what the cause-effect relations are in those circumstances. Um so I think that uh

generically in our physical theories, th there is part of the theory that tells us, you know, what what the causal mechanisms are. Um, I think it makes sense to imagine causes always proceeding from past to future. Not uh in in the other direction. So I d I don't see any reason to depart from that. Um and uh

Yeah, so i i in other words, um I think causation is a fundamental primitive of descriptions of physical theories. And even though you're right that, you know, a lot of people, including some famous people, have said ca causation doesn't belong in physics. I disagree. I it seems to me that it's a mistake to uh try to exclude notions of causation from physical theories.

For me it's quite the opposite. So since the late eighties and nineties, this subfield of statistics c called causal inference came along and they've done a quite a nice job of actually adding some mathematical formalism that allows you to talk about causation. And what's remarkable is is how well that slots into puzzles in physics. Right. So so like in in quantum theory in particular, I realized that the ability to talk formally about causal relations.

actually helps very much in sort of understanding what's going on in physics. And so I to me it was a big mistake to try to say that causation is is not a part of physics. Uh I think it is, and I don't see any reason why we should um go in for backward in time influences or any kind of exotic notions of causation. Right. Right now it seems to me that I think we can get away with very conventional notions of what causation is. Uh

while while still being required to innovate them a bit to to be able to cover the the quantum case. So uh in in other words, like, you know, when I think about trying to explain the Bell experiment, I think I can get away with a common cause. I I don't need anything exotic in terms of what's the structure

Uh where you do need something exotic is that you're not gonna be updating your knowledge by Bayesian probability theory. You're you're gonna have a more exotic formalism for doing uh that sort of thing. You said you think you can get away with a common cause?

Yes. Why not you can? Why why do you think it? Uh because I I so I I don't have an interpretation of quantum theory on the table to s you know, to tell everyone this is how you should think about quantum theory. It's it's a research program. Uh So there's there's principles that I think I can secure in a realist interpretation of quantum theory. Uh but we're not there yet. So the the the situation is that I think there's a lot of evidence in in favor of this being the right direction.

Um but we're not at the end yet. We don't have a story that cleanly separates, you know, what part of the quantum formalism are describing reality. How do we describe our knowledge of that reality? We we don't have that. So I d I like to say that it's

The the existing interpretation of quantum theory are sort of like a built house, but I look at them and I think they're all built on shaky foundations because they're all non-Libnitsian in one way or another. What I've got is is only the foundation, but I think it's a secure foundation and we're we're trying to build up the rest. It's a great analogy. So my understanding of causal inference is that Pearl? Yes.

Pearl's work? Okay, so perhaps I didn't get deep enough into Pearl's work. Perhaps it just started out this way and maybe it's evolved. I remember there were two variables. Mm-hmm. But in fundamental physics there is no do variable. Now operationally, yes, sure. But it's a do seems to imply an agent, at least to me. So has the theory of causal inferen inference moved beyond do to something more fundamental or something that doesn't require an agent.

Um let me try to answer at a kind of more general level and then we'll come back to this, which is I I would say uh the definition of causation in in the field of causal inference doesn't need to appeal to intervention. It doesn't need to appeal to these due operations. Like I was saying a moment ago that I I think you can define causation

Um, but nonetheless, when we're talking about, you know, what evidence do we have? What can I infer about what's going on Causley, often we want to avail ourselves of the fact that I intervened and this is what I saw. And and clearly there is a difference. about, you know, the probability of recovery given that I made someone take a drug or a placebo versus the probability that they recovered.

merely after having observed that they took the drug, you know, and a and not in a randomized controlled trial, right? Because in in those two cases I could have very different inferences. Um, so it it makes sense to talk about the agent in terms of like what you can learn. More broadly I would say You know, discussions of agents.

tend to leave many physicists feeling like, oh, this this all feels a bit new age, like consciousness is important or something like that. And and I don't go in for that sort of stuff. Um but I I think that's really the the wrong attitude. That the way I would put it is that There's a long tradition of pragmatism in physics. Uh if I look at the second law of thermodynamics, you know, where did it come from?

It came from Carnot studying heat engines and asking, you know, what's the the most energy I can get out of? uh a a steam engine. So that's a very pragmatic concern. It's it's about you know, it's it's about achieving the most efficiency we can. And and ultimately that led to the second law of thermodynamics, which is, you know, very fundamental in physics and thinking about the evolution of the universe and the evolution of life and

in places where there are no agents and no steam engines, you know, it it has applications there. We came to it by asking questions about not, you know, what just happens if I set up these initial conditions and let it go. But rather what are the fundamental limits on what I can achieve of you know, uh for a certain type of thing if if I'm really trying to engineer it to do as well as possible?

But that's also something that's constrained by the laws of physics, right? So so like the the limitation on the speed of any influence could be thought of as a limitation on how quickly I can send signals.

you know, limitations on how much work and I I can extract from some heat baths tells me something about the laws of physics. Even though it's about these sort of parochial concerns of mine as to like how much energy I can get, it can still tell me something really important about the laws of physics.

So similarly when we start talking about a theory of inference, a theory of knowledge, you might say, Oh, that's there's an agent there. But you know, look look at uh thermodynamics. So there was a time when we had various phenomenological thermodynamic laws and and we didn't really understand them. And then statistical mechanics came along. So people like Boltzmann had the kinetic theory of gases.

And they recognize that we don't have knowledge of the microscopic configuration of all these particles. So what we're gonna do is imagine a probability distribution over all the possible physical states, representing our ignorance of what's going on. And then we're gonna ask, okay, how how does that probability distribution evolve over time and and uh you know, what does it allow me to infer about various macroscopic quantities like temperature, volume, pressure, things like that?

But all along, there's an acknowledgement that we are ignorant and we have to model our ignorance quantitatively in physics to describe these situations. So it's not as if um, you know, talking about quantum states as states of knowledge is something highly innovative. It's it's been there in StatMek forever. We're always talking about probability distributions and our ignorance of the microscopic degrees of freedom.

There's nothing about it that is sort of new agey or appeals to consciousness. It is just being, you know, uh quantitative about your ignorance, right? So using the formalism of Bayesian probability theory to talk about your ignorance, uh, and doing it uh quantitatively. So i it's it's really the reverse actually, it's ironic because when you try to re uh understand the quantum state as being something real

you you often find that you have to go in for some idea like, oh, consciousness collapses the wave function and learning about something late influences the past, you know, like in a Wheeler's delay choice experiment, that that sort of thing. And it's only when you properly formalize, you know, what it would be for a quantum state to represent mere ignorance that these things become very uh conventional. Like if if I learn something in the future, I might update my knowledge about the past.

That that's not exotic or new agey in any way. It doesn't involve consciousness. It's just I I I find some fossils in the ground and I infer something about dinosaurs in the past. I haven't caused dinosaurs to exist. I've updated my information about dinosaurs. So a l a lot of the odd things in in quantum theory or things that people point to as mysterious. Uh, when you bring the agent in and you describe their ignorance, they they become much more innocuous. Uh

Times arrow. Does that presuppose causation or does causation presuppose times arrow? Or are these independent? Yeah, I think the the view that causation is a a primitive notion in physics basically presumes that there's an arrow to time. Yes. Um The situation is as fall. So people often point to the fact, like let's take Newton's theory as an example.

You know, here here's evidence that people will sometimes point to against time zero. They say, look, if you give me the configuration at, you know, time zero Uh then it will completely fix the configuration at some later time. Just evolve things lost forward. But similarly if you give me the state at the final time, I can evolve it back and get uh the situation at time zero. So there's this time symmetry there. And so there there doesn't feel like any asymmetry.

And so that means that, you know, what why should I say that there's uh an asymmetry between the past and the future? Um to me that that feels more like a fact about inference. Right. So just like you know, inferring the existence of dinosaurs from fossils, in Newton's mechanics, if you tell me the configuration of particles at one time, I can infer what the past was.

And you know, and I could also infer the future from the past. But if you ask me, uh if you imagine a change to the configuration at one time. Uh what what do you think that that means? And and there I think the intuition is strong that like a a change at this time implies a change in the future, but it does not imply a change in the past, right? Because that's

the the the conventional way we we think about causation. Certainly like when we go through the world and we intervene, we understand that our interventions are gonna have effects in the future and they're not gonna have effects in the past.

So I I like to think of the the the standard arguments for time symmetry as really being arguments for time symmetry of inference, right? Yeah. You know, the the theory of inference doesn't care whether you're making inferences from the past to the future or the future of the past. Uh, but I think causation does care. And and because I think it's a primitive notion, I think there is an arrow of time in our physical theories.

Since we're talking about the philosophy of physics Often philosophers of physics will say that many physicists don't have a conceptual understanding of of what they're speaking about, whether it's GR or most of the time it's quantum mechanics from my conversations on air and off air. So what does it mean when someone understands something conceptually? And how does one go about if one wants to increase that muscle, how does one increase their conceptual understanding muscle?

Hmm, that's a that's a really good question. Um Yeah, uh okay, so let's see. Certainly I would say from the kind of quantum foundation's perspective. you know, increasing your conceptual understanding i i is about, I think understanding what's going on beyond just saying I can predict what's going to happen. So there's this kind of operationalist point of view, which is all you should ask of a theory is that it makes predictions. Um so I I think that's a very poor

image of science. Because if you gave somebody an oracle that would just answer yes no questions, you know, it it would serve the same purpose. Like I could I could ask it a question and, you know, is this thing gonna happen if I do this experiment? Yes or no? It answers it. I get everything that an operational says a scientific theory should give me. But the problem is that's not really what I want out of a scientific theory. So if I wanna cure cancer

I I don't even know what question to ask, right? I don't know where to start. I I need to have a model of like what what is the Wh where do I start on this problem? Um and I I can't just reduce that to answers to yes, no questions. It's because I have like a a uh conceptual image of of what's going on. And so in other words, scientific theories don't just answer questions, they provoke

questions, right? They they they tell you what the next question you should be asking is. So People who have different views on the interpretation of quantum theory tend to ask very different kinds of questions in their research.

They they tend to predict that we'll see, like if if they happen to believe that there's gonna be a breakdown in quantum theory at some point, they'll predict where that breakdown's gonna happen very differently because they have very different conceptual understandings uh of what's going on, uh beyond what an operationalist would say. Uh so I think yeah, c conceptual understanding

You know, that part of that is could be, you know, having a realist interpretation of of what's going on. Um you know, often it in a in a smaller context, like when when you're looking at some phenomena, uh And you sort of, you know, you follow the derivation and you say, okay, I guess I I I sort of understand where that result came from. But you know, to to really understand it. Uh like I'll often tell my students like If you can show another example of this phenomenon in a different context.

Well th then you're onto something. Then you know you've got some you've generalized the idea from the context in which you saw it to a broader context. That indicates some understanding. So yeah, often like generalization is a good uh proxy for a deeper understanding.

Um yeah, sit you know, unification situating this phenomena as cognate with this other phenomena or another example of something, that's also important for understanding. So I think there's there's a lot of um signatures that, you know, you you you don't just follow the derivation, you really understand what's going on. Something I want to do with this channel and I I hope I have been doing and I hope to do is to have a

positive influence on the research scene in the places that I that I inter in the on the themes that I interview about. So whether it's biology with Michael Levin or developmental biology in that case. or the philosophy of physics, for instance, with you, Jacob Barnes, Tim Modlin and Sean Carroll and so forth. And one of the ways to have a positive influence is to articulate something that hasn't been

Said much to the public. So the public has heard from people like Neil deGrasse Tyson that philosophy, maybe even the philosophy of physics, but Let's just say philosophy is is a f uh sure it was useful in the past and sure it maybe gave rise to science. In some sense, natural philosophy became physics, but then at some point it it loses its ROI. And I'm sure you've heard similar, I'm sure you've had, even privately, perhaps even publicly, arguments about this.

Can you make the case for me? Make my life easier? Why should we not discount the philosophy of physics? What is it providing us? Well I I th I I would say um maybe rather than focusing on the academic discipline, like the departments of philosophy that employ philosophers of physics, let's kind of focus on the

the culture of philosophy of physics. So so there's people in physics departments who who are doing philosophy of physics, right? So often people studying the foundations of quantum mechanics are are taking a much more philosophical approach to physics than typical physicists do. So I want to maybe defend that methodological stance, which is like, let's ask the kinds of questions that are typical of natural philosophy. You know, what why is that useful to do in physics?

And and I think the Again, so this like when we're talking about methodological questions, i i you know, it's not as if evidence data will s solve this, but but we can look to history and say, well, what sorts of approaches have been beneficial in physics? And so if I look at the really big revolutions in physics. Um they all came along with, you know, they they were instigated by people that were thinking very philosophically.

And they also often came along with radical changes in how we even think about science. So they a lot of revolutions in physics also meant revolutions in the philosophy of science. Okay, so you know, Newton and Einstein in particular with relativity and quantum theory, I I think that how we even thought about what a scientific theory is changed as a result of what they were doing. And of course they were very interested in philosophical questions.

So um, you know, on the other side is when you when you look at some of the best progress, you know, in the in the twentieth century and more recently, it was sort of uh technical sophistication that led to that progress and not thinking about

uh, you know, deep questions about, you know, the nature of uh locality or relationalism or or anything like that. Um and so that's sort of the the kind of the bias we might have uh these days in physics department is that, you know, so some of the recent progress, you didn't need to think deeply about philosophical issues to make progress.

Uh but what I like to tell students who who maybe come with that attitude and say, Oh, I d I don't know why foundations of quantum theory is is so important. Um I would say, well W when you compare the magnitude of the revolutions, you ask, well, what are the revolutions that were like really significant? Uh they're the ones that I think touched on deep philosophical issues.

Uh that that's my sense of it. That, you know, revolutions in physics that, you know, change what the nature of space and time are, you know, that that philosophers have talked about the nature of space and time for a long time. That's a very philosophical issue. Uh i I think they are more interesting, more significant. uh than mere technical innovations. Um so yeah, I I guess I would just say to students that, you know.

It's good to be philosophically savvy if you really want to be part of revolutionary physics, right? You don't need to be philosophically savvy if you want to do good physics work, but revolutionary work in physics tends to require that kind of savvy. Um the the other thing I would say, so that's sort of maybe one a kind of answer. The other kind of answer is just that

You know, I so I I did a degree in philosophy. I did a joint degree in physics and philosophy during my undergrad. And what I say would say I I took away from my philosophy degree is not the details of any particular philosophy that i that's not what was important for me later. I would say in philosophy there's a certain discipline. about what constitutes a good argument. So so you learn in a in a philosophy training. basically is if you're doing analytic philosophy as I was uh

how to make an argument bulletproof, right? So how how to, you know, define your terms carefully and and how to explain things well and how to see the errors in your own analysis and how to go looking to criticize your your own thinking and uh yeah, there's there's a certain way of approaching

argumentation characteristic philosophy. And at the time when I was doing both, I noticed that in physics everything was much looser, that, you know, oh, we're gonna make these approximations, not worry too much. Let's try to get an answer. there there was a lot less uh rigor about, you know, is this argument a good one? what are the premises that this argument ultimately rests on?

you know, can I articulate better the principles? Can can I, you know, dispense with some of the axioms that I'm using or the principles that I'm using, this argument. Like there's a lot of things that in philosophy you you tend to do to to try to get to the essence of something. So you sort of learn to try to get to the essence of something. That

I think good physicists do that as well, but it's it's less a part of the training of a physicist, and it's much more the part of a the training of a philosopher. So I think there's also just some skill sets from philosophy that translate well to the foundations of physics.

So you did your undergrad at McGill, correct? Yep. Right. Okay. I didn't even know they had so I did mine from the University of Toronto, which you did later for your master's and PhD. Yes. Okay. I didn't even know McGill had a philosophy of physics undergrad. Well, uh no, they they don't. So um The situation was so I I started at McGill in physics. Um And uh even though I was sort of

You know, my my loves were physics and philosophy at the time. There there wasn't really that option. But I met somebody in my first year at McGill. And he had done most of a physics degree, but, you know, wanted to change course and he'd become interested in philosophy. Uh and uh he he noticed that McGill offered a joint honors degree in philosophy and English.

philosophy and political science, philosophy and linguistics. Like philosophy uh gave you joint degrees with almost every other discipline, but there was no philosophy and physics. Uh and so he went to the faculty of arts and science and said, why can't there be a joint honors degree in philosophy and physics?

Uh and so they went to the Department of Physics and said, Will you relax your credit requirements to set up this joint degree? And the physics department said, Absolutely not. There's no way we can relax any of these credits. Uh and so then this guy went back and said, Well, what if I did it in five years rather than four? And the physics department said, Yeah, that'd be okay.

So they they made this five year program that was basically the physics curriculum, the whole physics curriculum, together with what was effectively a philosophy of minor. And so when I learnt that that thing existed, because he'd basically created it, I was like, Yeah, that's that's what I want to do. So I I enrolled for that. Super interesting.

Okay, now you mentioned analytical or analytic philosophy. Was there anything from continental philosophy, which I'm sure you had to study some, that influenced your physics? Not really. No, I I I didn't have to study much continental philosophy. Um I think I naturally gravitated to the analytic philosophy courses because uh they were sort of

more clearly just yeah, useful and and in line with with how I was thinking. Also there was uh a professor in the philosophy department, his name was Paul Petrovsky. He was an amazing lecturer, um, and for whatever reason he he taught So many different courses while I was there. So he I took his philosophy of

logic and philosophy of language and epistemology and philosophy of mind and a a long list of courses and and they were great. So uh yeah, I didn't actually have that much room for many more courses. Um But uh it was it was it was a great uh degree. I I also got exposed to for the first time to the foundations of quantum theory in a seminar course on metaphysics.

So it was the philosophy department where that kind of ignited my interest in the foundations of quantum theory because the seminar course basically we were reading primary material on a on things like the quantum measurement problem and Bell's theorem and I remember you know reading these things and then going back to my physics professors and saying, look, there's a real problem here

And uh they wouldn't acknowledge that there was a problem and they told me that I was mistaken and I just didn't understand what was going on. So yeah, the physics department uh really tried to persuade me not to to work. on these kinds of issues. Uh, but it was too late. I was already kind of hooked by virtue of my exposure and this seminar course. What surprised you in the philosophy of language course? Uh that was a long time ago.

Yeah, I I I thought it was interesting, the whole kind of analysis of language turn in in analytic philosophy. Uh Yeah, I mean it it it had elements of kind of logic. You know, when you when you take a a an ordinary language English sentence and you try to decompose it into a uh something that's more logically precise.

you know, there there exists a person such that that person is the king of France and they are bold, you know, like that. That those kinds of exercises uh you know, they're they're a small part of You know what? I think is is useful to do, which is, you know, often we'll have ideas and we'll try to express them clearly

and and being able to sort of parse English sentences into things that are more precise and get clear on exactly what you're trying to say. You know, often the things we're trying to say in a paper on the foundations of physics are quite nuanced. And so you you You wanna use mathematics, like mathematics is a great tool for saying things very precisely.

Ordinary language is a is another great tool, especially if you want to be understood. And in between there's sort of a mix of, you know, some signposts of ordinary language and then some formalism that is maybe the most effective at sort of really communicating. And so I think a lot of those courses and the logic and uh philosophy of language helped me a little bit in terms of like h how do you make arguments understandable.

Now I wanna understand Rob more. Okay. And I want the viewers to understand Rob more. So now would be a great time for you to take us through what did you used to believe as an undergrad? How did that change as a grad? How did that change as you became a seasoned researcher? So take us through that timeline. Okay. All right, so as an undergrad, uh that's when I first got exposed to foundations of quantum theory. Uh I became interested in it. And

I really wanted to work on that as a as a grad student, but there weren't really many opportunities to study, you know, the foundations of quantum theory. Um So I went to the University of Toronto for my master's and PhD and uh I started out as a high energy experimentalist for a short time, and then I realized it wasn't for me and uh I I looked into, you know, things that things I could do that were theory and uh There was a prophet there by the name of John Sype.

who had a student who was working on decoherence theory and so it was sort of adjacent to foundational problems in quantum theory and Sype? Yes, John Sype. Yeah, yeah, I remember him. Did he also teach the combinatorics course or you have no idea? He d I don't think he taught a combinatorics course. So he was in the physics department and

For many years he taught a course on the interpretations of quantum theory. Um so he he was interested in the foundations of quantum theory. And uh so I I remember You know, re realizing that th there's an opportunity here Um, I could I could work with John, do the foundations of quantum theory, which is what I really wanted to do. But I knew that it could very well end my career in physics because I knew there weren't really any

real prospects, you know, there weren't faculty jobs in the foundations of quantum theory. There might not be many postdoc opportunities either. Uh and so I had to make a tough choice'cause I I could have done something sort of more conventional and sort of bided my time and uh you know, w when you have tenure you can do what you like. Versus you know, do do what I was interested in right right then and uh

risk ending my career. So in the end I decided I would work on foundations of quantum theory. Uh I sort of made my peace with the idea that this could very well end my career in physics. Um and You know, m mostly it was I d I I thought I wouldn't want to spend you know, you the way I I say it is that you you you need a lot of motivation. You know, s success in in

theory and in physics is often comes down to can you, you know, do the hard work?'Cause there's a lot of hard work involved in making progress and and you have to be motivated to do that hard work. So if you're working on something that you really care about, uh, you're you're motivated to do that work. So Um, I decided, yeah, I'm gonna I'm gonna do foundations Guam Theory. And uh just as I was graduating, uh Perimeter Institute opened.

uh and that's just down the road from Toronto and Waterloo. And they were, you know, w one of the founding fields. So there was only four fields representing a perimeter in the early days. It was uh uh quantum gravity, fields and strings, quantum information, and foundations of quantum mechanics. So they were looking for postdocs who had expertise in the foundations of quantum theory.

And the situation was that not many people in the world had described decided that they were gonna, you know, risk their livelihood in physics on on studying this stuff.

So uh there wasn't a lot of competition, I would say, but it it fit very well. So I you know I could move into a postdoc position at Perimeter. Um and that's so I would say uh the the place where I became so I I worked with John on Uh we we were he he gave a course on interpretations of quantum theory, so I had familiarity with all the different interpretations of quantum theory and I and I was dissatisfied with all the existing interpretations.

Uh now a friend of mine, uh his name is Joseph Emerson, so we did undergrad together at McGill. He had gone to do a PhD with Leslie Ballantine, who's at Simon Fraser University in uh British Columbia. And uh Valentine famously has a textbook on quantum theory and he uh also has uh some articles arguing for what he calls this the uh

I think it's the ensemble interpretation of quantum theory. Anyways, it i it's very similar to this view that quantum states represent states of knowledge. Um, so my friend Joe visited me in Toronto and and his project uh with Ballantine what was really pointing out that a lot of features of quantum theory make more sense if you make the analogy between quantum states and probability distributions over classical states.

So they were looking in particular at the air in fest limit. So the idea that, you know, I I can recover a particle trajectory by the sort of expectation value of the position. And you know, in situations where let's say you have a wave packet and it and it hits a barrier and it splits into two, the expectation value of the position goes right through the center of the barrier. And that kind of doesn't make sense in terms of like that particle picture.

But if you think of that wave function as just analogous to a probability distribution, then you can say, oh, well, with some probability the particle goes left, with some probability the particle goes right. You have a very similar phenomenology for distributions over phase space. they compare much better to wave functions than individual trajectories do. And they were studying you know these kinds of questions for chaotic dynamics and and he had a sort of body of

evidence that we should think of quantum states in this way and and uh at the time I wasn't persuaded but I thought that If this is wrong, uh this evidence needs to be explained away somehow, right? Like that there there has to be if it's wrong to think that quantum states are epistemic, then there's some reason that that these uh phenomena suggest it, but it's but it's fundamentally wrong.

Uh and so then around the same time, so this is like late late in my PhD, uh, I started hearing talks by Christopher Fuchs at at conferences, and uh he is one of the main proponents of quantum Bayesianism. Well, not now it's called Cubism. At the time it was quantum Bayesianism. And so he also was uh providing evidence. for the correct interpretation of quantum states to be states of knowledge or states of belief.

And so the the evidence started mounting, so th I felt like there was more things that You know, I I felt needed to be explained if the quantum state wasn't State of knowledge. But then I kind of got interested in in just trying to figure out for myself whether other phenomena could be explained with this view of quantum state. And suddenly I s I was able to sort of add to the list of quantum phenomena that you could naturally explain in this way.

And so I I started changing my own views. So I I had worked a lot on uh interpretations of quantum theory where the quantum state was describing something real. So, you know, things that were a bit like Bohmian mechanics. They were called modal interpretations. I'd worked on those. Um but I yeah, my views were were shifting. I was starting to see, you know, the evidence and and it was compelling.

And then what was very decisive was I went I think it was two thousand and two, I went to a conference in Oxford on the philosophy of physics. And I was talking to various people about, you know, what isn't this interesting that there's all this phenomenology that can be reproduced if you take the view that quantum states are like probability distributions and

Doesn't this need explanation? Uh, and what do you think? Like how d how do we explain this? And i everyone was very dismissive and they saw No, that's just clearly wrong and it can't explain this and it can't explain that and and I came away very sort of frustrated at the dismissive attitude that people had and

And so I said, Well, they say it can't explain this, that and the other thing, but I think it can and so I remember I I sat down after the conference, you know, in the airport to to work out how to explain it.

And on the flight back I basically worked out the the what became the toy theory. Like all the I like started I think with the quantum teleportation protocol. I showed how you could explain that in one of these classical theories. So that that that was sort of the moment where I saw, Oh, okay.

really works. And it was very um impactful in the sense that all I put in was this idea that, you know, there's a classical physics and and your maximal knowledge is not complete, but but everything else is just conventional classical physics. And out came all this phenomenology that was uh you know, recognizable as things that you see in quantum theory and quantum information theory.

And I hadn't put that in. I just like every time I would ask a question, like, Oh, is there a no cloning theorem? The answer would come back, yes, there's a no cloning theorem. Super interesting. And so, you know, once you got a like a long list of things that you haven't put in and and they're just coming out and they look very much like uh the quantum phenomena, you say, Well, there's there's something right about the principles that went into this toy theory.

And uh so for me that was like i the this analogy between pure quantum states and states of incomplete knowledge, like that struck me as the the key innovation of this sort of approach relative to things like Bowman mechanics and other hidden variable models, which were not like that. Um so that sort of set me down this.

you know, path of trying to pick out what aspects of the phenomenology really are surprising. They can't be explained in this way. So so that's, you know, I was thinking about contextuality a lot, Bell non locality, and and trying to understand well what what else do we have to vary uh what what new principle do we need to really reproduce quantum theory? Um and yeah after coming to perimeter So uh I did I did a post docket perimeter. Uh that's when I wrote the the toy theory work. Um

I also worked on contextuality and then I went to the UK. I had a a Royal Society fellowship and I spent three years at Cambridge um in the quantum information group. And uh yeah, I would say You know, f for for many years I was struggling to find that kind of extra

principle, some some direction as to how to get a proper realist interpretation of quantum theory. And it was only when I came across the field of causal inference and I started You know, I I had always been interested in sort of unscrambling the omelette of what's about. ontology and what's about epistemology, the the toy theory sort of pulls those things apart.

But I started thinking about the ontology from this causal lens, uh and the epistemology and as a theory of inference. And I s yeah, so eventually I realized, okay, I think this new framework uh that comes from the Cosmos community is is really the right way of thinking about uh quantum theory and it's an innovation in the language of this framework that I'm really looking for. So so that's sort of

I mean there's maybe a lot of other details of the story, but in terms of my views on quantum theory, uh that's how I got to where I am today. What's it like being at the faculty at the perimeter? Tell me about the culture, the ph the philosophy of physics culture and and how it's viewed, how you are viewed, and how the collaborations are like, and tell me about it. Yeah, it's it's very collegial. Uh we we don't have philosophers here per se. Uh we have physicists.

I I would say the the thing I appreciate the most about being at Perimeter is that you know it it was it started with very much the vibe of a a startup, very ambitious and You know, from the very beginning the idea was that, you know, we we want breakthroughs in physics, we want to revolutionize physics, and in particular Lee Smolin uh created this culture. Like if you spoke to the Lee, you know, especially in the early days.

you know, that that's our objective. You know, we we need to work on the big problems. Uh we we need to revolutionize physics. And I think having that as your mission is very good because it it uh there's a temptation always to sort of do the thing that you can do.

because you can get some some result and and you know, you might drift away from the really significant questions. But you know, if if there's this constant reminder that no, you're you know, you're you're supposed to be working on the really tough problems. then you're more disciplined. Like you might still work on smaller problems, but it's all in the service of this bigger goal. So I definitely feel that, you know, that so I would say uh my own research program

is as ambitious as it is, probably in part because I'm here and uh that was kind of the DNA of this place is that we should always be ambitious. So I appreciate that a lot. Uh It should also be said that because Perimeter supported the foundations of quantum theory from the very beginning, which was uh the year two thousand You know, it was one of the v very few places in the world that was doing that.

Um, so you know, back then y y you didn't find many groups in the world working on the foundations of quantum theory. And as a result, you know, over over the years. it's been a great place to support the foundations of physics, the foundations of quantum theory. Uh we've had, you know, as visitors all

kind of representatives all different views come through. So you get to talk to the top people in the field on a regular basis'cause they come through. But you can also attract uh really talented postdocs and and students. who are interested to come. So so you have a really high talent density.

Uh, which is obviously, you know, any individual researcher benefits a lot from having really smart people around who have thought deeply about these things. So uh yeah, I'm very appreciative of that aspect of perimeter. As I said, I mean, the... Uh the different groups of perimeter are are are very collegial. You know, we we sometimes uh collaborate together. Uh especially like the closest groups to quantum foundations would be the quantum information group and the quantum gravity group.

Uh so yeah, we have the strongest interactions with those two groups. Uh that works out very well. Um yeah, I I would say uh it's it's a really good place to do uh the foundations of quantum mechanics. It's it's one of the bigger groups in the world, right? So I mean there there are some that are bigger, but uh it's it's big enough that generally we we kind of

n um have a sense of sort of e everything that's going on in the world in Quant Foundations. So if there's some new result, we somebody here will uh inform themselves about it, we'll have a talk about it. So so we can sort of be up to speed on what's happening in the field. And so it's it's nice to have that overview of the field.

you know, ra rather than becoming very focused on, you know, your own particular thing, which which can happen if you're in a a smaller group, you you get a sense of everything that's going on and what what are the the the big trends and the the ideas. And uh so yeah, it's it's a really nice place to if if one is interested in the foundations of quantum theories to sort of s see what's out there, understand what what's going on.

And what makes it different than other physics departments or physics institutes? The perimeter institute. Well there are no labs, it's so it's a theoretical physics institute, so we don't have experimentalists at Perimeter. Some of us do work with experimentalists. So I have an experimentalist colleague at the University of Waterloo, which is just down the street. And I work with his group uh doing quantum foundations experiments.

You know, th the origin you know, the founding, you know, the i institute was set up by Mike Lazarides. He gave most of the money to set up the institute. It nowadays it's funded partially by the federal government. partially by the provincial government and partially by private donors. Um I can give you some context behind my question, just just just so you understand my mind frame. Yep. So Mike who you who you referenced, Mike Lazarita.

He invited me recently a few months ago to his home. Beautiful. I don't know if you've ever been, but it's the most beautiful home that I've ever seen. And he sent this private driver to come pick me up all the way from Toronto to this place. And I was speaking to the driver and the driver said, He's like, Bro, you have to see this play. You've never been, you have to see this place.

The uh anyhow he just kept going on and on at saying that, oh bro, I I've driven for Drake, I've driven for this, but this guy's home, he didn't even know the guy's name. He said this guy's home tops it, and uh so I was so hyped. Anyhow, I spoke to or or Mike wanted to to ask me about a a variety of topics, my opinions on them. Something that became clear to me was

that many of the faculty at Perimeter have expressed to Mike that there's been a change in the culture over the past twenty years. And I just wanted to know if you've seen a change in how I don't know how the foundational research is done and how well, I just wanna know if you've seen a change either the positive, neutral. It I just wanna know what is it like? Well I think that maybe the biggest change over the years is that permanent institute

by virtue of being very successful ha has grown a lot and any institution necessarily kinda changes as it gets much bigger. So in the early days we we didn't need a lot of kind of rules and policies and things like that because it was very small and everything could be decided on a kind of one off case. So it really had that feeling of a a start up and

you know, with with v very few people and a large endowment, uh uh uh y you have a lot of flexibility in terms of what you want to do. And then you become mature and and now you you have uh You're doing a lot more science, but it also comes with um you know, challenges just in terms of administering a a a large institute. So I would say yeah, that the

maybe the most noticeable changes are are are just the kind of standard changes that come from any institute that grows from something small to to something much larger. Uh I still feel like there's the that we haven't lost our DNA of, you know, we're we're here to make breakthroughs and um we have to stay focused on that. Uh So i we've certainly expanded the number of fields of theoretical physics.

So as I mentioned earlier, you know, it started out with just quantum foundations, quantum information, quantum gravity. And uh quantum fields and strings. And and since then we, you know, we now have a condensed matter physics group, a particle physics group, a cosmology group, a strong gravity group, a mathematical physics group. Right. So it it it it's expanded in terms of the the scope of areas of of physics. Uh, which is good because, you know, there's connections between

all these different areas. Um but yeah, it it also means that, you know, in in the early days, if you wanted to, you could go to every seminar and there's no way anyone could uh attend every seminar anymore. So you have to pick and choose uh what yeah, well who who you're going to interact with. So it's a good problem to have. Is there a piece of advice that you consistently give your students?

Yes, I mean there's many. Uh Maybe on the on the side of being understood, I I think what it's it's a particular challenge in one foundation. Because many researchers come with with de very different ideas, it's sometimes hard to communicate effectively. So so it's It's really important if you wanna get your idea across to to learn how to communicate effectively. And so

It's it's I tell them, you know, it's it's all about the narrative, right? That you you need to have a strong narrative. So if you're giving a talk, you need to like make it really clear what you want people to come away with. So or when we're writing papers. Uh so there's a lot of things. Can you give an example of that? An uh an example of how that's done poorly and then how you would sharpen it, tighten it, improve it.

Yeah. Maybe something that that would be done poorly is like if I am making an argument for something. And I just sort of jump into here's here's a fact. Here's another fact. Together they imply this and they imply that and we arrive at this result. Uh

that's worse than, you know, here's what we're interested in and and here's a result. Like it here's a theorem. Now I'm gonna tell you everything you need to understand what this theorem says. I'm gonna give you the definitions of the term so that you understand the content of the theorem.

before I try to persuade you that it's true, right? Or before I try to give you the proof. And often in talks, you know, uh trying to give the proof is is definitely the wrong thing to do because the audience won't have the time to absorb it. But Spending the time so that they understand what the result is saying, what the definitions of the terms are. That's really what you you want to focus on. So you want people to be able to come away and say,

I understood what the claim was, you know, they claim to have proven this theorem. Uh I have to go to the proof to to make sure that it's right, but I understand what what they're claiming. So things like that where it's about um

what you should be emphasizing, what you should be spending your time on. You know, spend more time on motivating it because if people don't see why what you're answering is is interesting, they're not gonna pay attention. So you gotta spend a lot of time motivating why you're trying to answer this question. That's extremely important and you you don't want to try to contract that. Uh so advice like that, I feel like that's the kind of advice I'm

Uh, I feel comfortable giving uh very kind of small scale advice about how to present well, how to write well. I I actually did give a talk to the grad students on how to give a good talk and I've given talks on you know how to how to write a good paper. So this is something I do think a lot about, like what are the tricks you can use to write well, to present well. Um I I also sometimes offer advice about sort of like how to do physics, but there, you know, it's harder because

different people with different strengths are are gonna make progress in different ways. So there isn't necessarily sort of like one kind of route to making progress. You know, people who have different styles of research, that that can be very useful. So you don't necessarily want to force somebody to do research in any particular style. Are you often misunderstood?

I think so, yes. Yeah, it's surprisingly so that uh I it it is a frustration in a way that I I I spend a lot of time and energy For example, write writing papers in a way that I hope, you know, that this cannot be misunderstood and giving talks where I hope I'm sort of clear about what what I'm arguing for.

Uh but yeah, I will often learn that, you know, peop people don't understand what what I'm arguing or summarize my views in a way that's surprising to me because it's sort of the opposite of of what I believe. Um so yeah, I'll often In in trying to like uh the the students, for example, coming back to the students, like uh they will complain about how long it takes to write a paper with me because

I can always see opportunities for improvements and places where we could be clearer and and I often argue that, you know, in spite of putting in so much effort to be clear and and going over this text many, many times, till it feels like

it could not possibly mis be misunderstood. Nonetheless, it it will be. Uh so they get that that's the one of the mottos I like to say is that uh don't write so that it's possible to be understood Right so that it's impossible to be misunderstood, you know, that that it has to be totally clear what you're saying. Um, and even when you set that as your goal, you're you're still gonna be misunderstood. So if you don't set that as your goal, you'll definitely be misunderstood. Um I see. Yeah.

So why don't you give an example? What is it how about this? What is it that Rob is most misunderstood about that frustrates him the most? Like the the intersection of those two. Okay. Um Yeah, so we're not going to be able to So so I think if so a lot of my articles Are set in what here I've called the sort of conventional framework for realism, or the ontological models framework, a kind of classical view of how to be a realistic.

And I I you know, so my toy theory is set in that framework. And I derive kind of the all the things you can do and and and the limitations, like what you can't do. And and I do that because I'm studying it as a foil. I hope that it'll teach me something about quantum theory. And from the very beginning, you know, I I I've I've noted that you know I I'm not proposing this as the correct interpretation of quantum mechanics. This is just

meant to be an argument in favor of a particular interpretation of the quantum state, but the correct interpretation of quantum theory is is not going to be this sort of thing. Uh in fact, I think that we have to abandon this framework and come up with a new framework. Um but that's a sort of a a a subtle point. uh that, you know, you might study something that you know to be wrong to learn something about, you know, where you're trying to go. So, you know, the the analogy I like to use is um

If you look at Picasso's early paintings when he was fifteen years old, you know, he he painted in the classical style. He could paint like a master at at age fifteen. But I don't think Picasso could have sort of transcended the the norms of art if he hadn't mastered the orthodox approach, right? Like I think a lot of what he did required a mastery of the orthodox approach. So a lot of what I do is like, look. Um hidden variable models w weren't really uh we we didn't we don't fully understand

what they're capable of reproducing. If we take hidden variable models where we can have some limit on how much we know, there's a lot of phenomenology we can reproduce. And we should study that to know what the limits of what we can understand are.

Because that that will tell us, you know, anything that's beyond those limits is what is really surprising and needs to be understood. So I kind of study these as as a foil, as as something that will sort of prepare me for the more radical step that will get me to a proper realist interpretation of quantum theory. But often people think I just endorse this framework, that what Rob wants is a ontological model where the quantum state is epistemic.

And that's not what I want. I I wanna reject the framework of ontological models altogether. So yeah, that that's one of the things that I feel like I can never no matter what I say People will think that I'm endorsing this ontological models framework. I very much hope I did not misinterpret you at all during this conversation. I've been studying you for four weeks.

your work and then also other work that is related to yours and and dates stick out to me. So I know nineteen ninety four, Rohrlich, I believe, Rohrlich came up with a non local correlation bound larger than Bell's correlation. And then that The PBR theorem is supposedly excluding epistemic interpretations, but you have your responses to that. So it's it's difficult for me to to formulate a question that isn't loaded with a misunderstanding.

A loaded question may have a misunderstanding, so I try to ask the more Okay, well what is the PBR theorem? And tell me what you think about it. Rather than me saying, look, the PBR theorem excludes your theory because of reasons A and B, even though you've said C about it. That's n that's false because of D. Yeah. In fact, Rob, part of the reason that I do this podcast is so

I can understand or ensure that I'm understanding the person that I'm speaking to, the interlocutor's theory by recapitulating it. And then hopefully they agree. They say, yes, that's what I meant. That's what I said. And then it Only then that I realized I didn't misconstrue them. Otherwise it's super easy for me to rapidly misunderstand something, and I'd rather slowly understand something. So anyhow, I hope I haven't misunderstood you or misrepresented you.

What what do I say about the PBR theorem? So the way the PBR theorem is often uh understood and the and the way it's maybe the popular understanding of that is that shows that uh ontological models that are what I call psi epistemic, so I can come back to define that in a moment, are are ruled out. They cannot reproduce the predictions of of quantum theory. Um so okay, what it what does it mean to be psi epistemic? So um the I I I showed this toy theory um

I I I did further work and y you can find sub theories of quantum theory. So you can find So what do I mean by a sub theory of quantum theory? If I take quantum theory and I say, let me just consider some subset of the preparations and the measurements and the transformations that are allowed by the full theory, I I can get

sub theories, right? So so they certain they they have a lot of features in common with quantum theory, but they just don't span the full set of states and measurements and transformations. Um and so there's these certain subtheries, which I like to call Clifford subtheries, because the transformations uh that are reversible form the Clifford group. Um and they get studied a lot in quantum formation theory.

And there's continuous variable versions of them. But anyways, it's it's a part of quantum theory. And you can show that these things admit of Leibnitsian ontological models. So they're they're local, they're non-contextual. Uh you can often define a Wigner representation of the states and the measurements, and the Wigner representation gives you a probabilistic interpretation on a classical phase space for everything in these theories.

So it's another angle on how how do you understand what part of quantum theory is sort of easy to explain from a classical point of view. Well, these Clifford subtories uh don't have anything truly innovative um

about them. Now when when you think about those kinds of models, they're they're still in a way hidden variable models, because you're imagining that there's some ontology and some probability distributions over it, but they're very different from something like Bowman mechanics or any other pilot wave theory. And so um the the way I tried to articulate that difference in a paper was by defining a dichotomy for ontological models between psyontic and psy epistemic.

So, um Boehming mechanics is an example of a scientific model. These uh Classical theories of of the the Clifford subtries of quantum mechanics are examples of psi epistemic molds. So so what's the definition? Um And and why this terminology? So it it comes down to the following. If if I think of a variation of the quantum state.

In the full theory or in one of these subtories. Does imagining a variation in the quantum state force me to imagine that the real physical state of the world has changed? Or might it be consistent with nearly a change in my knowledge of the world? So so if, for example, um the quantum state itself, like if I think about the the

the quantum state, the universal wave function is sort of part of the physical state. And then there's these particle configurations. So if I say, well, what do I know when I know the quantum state? Well I have a kind of delta distribution on the axis which runs over all quantum states. And then some you know non-delta distributions along the other axis.

But but those distributions don't overlap. As I vary the quantum state, I move to a completely disjoint distribution, right? So there's no overlap between those distributions. Whereas the kinds of models I was telling you about earlier where sort of one quantum state gets represented by a distribution like this and an or non-orthogonal quantum state gets represented by a distribution that overlaps with it.

So now I can imagine a variation from this quantum state to this quantum state, but the physical state has not changed. It's in that region of overlap. So that kind of model means I can imagine variations in quantum state that don't necessitate a a variation. in the real physical state. So those models are psy epistemic. Okay, so that the quantum state must be interpreted as a state of incomplete knowledge in those models. So that that's the dichotomy.

Uh among the psyontic uh uh models, there are some that are psi complete, which means there's only the quantum state, there's nothing else to say. So that's like some orthodox interpretations of quantum mechanics that do not introduce any hidden variables are are basically those psi complete models.

And then there's some that I call psi supplemented, which is like, oh, the quantum state is real, but there's other stuff as well. So Bohmian mechanics is an example where there's these particle configurations that are real as well.

Um but being psi incomplete, you know, psi not being a complete description, that can happen with with psi being ontic or epistemic, right? One way in which psi could be incomplete is if it's merely describing probability distributions, and then there's some deeper reality below it.

So roughly speaking, the old debate, you know, the one that goes back to Einstein, Podolski, and Rosen is is quantum the quantum state complete or incomplete? That was the language they used in that paper. But but this newer debate is is it ontic or is it epistemic? So the um the the toy theories

And these classical theories whereas there's a restriction on how much you can know. To my mind, the fact that they can reproduce so much of the phenomenology of quantum theory gives evidence that it is better to think of the quantum state as being epistemic.

But strictly speaking, this dichotomy I just defined for you is all in this framework of ontological models. So, you know, I want to reject that framework, but I still think it's useful to study, you know, scientific versus psy epistemic ontological models. What can they explain?

So so that's the sort of spirit of all this, which is like ultimately, I don't think any of these models are the the right picture of reality. But by studying them, like kind of mastering what's possible, we can learn something about you know how to reject the ontological models framework and move on.

So I think, you know, Bell's theorem and the Cotian-Specker theorem, which are the sort two big no-go theorems on quantum theory, what in this language what they show is it doesn't matter if you're psyonic or psy epistemic. If you believe in local causality, you can't reproduce the quantum predictions. If you believe in non contextuality, you can't reproduce the quantum predictions. Being psy epistemic is not gonna save you. Um right. It's it it's just can't be done.

And because I think that the Leibnizian principle underlies local causality and non-textuality, to me, those no-go theorems are just saying Take the conventional framework of ontological models and the Leibniz principle, you're gonna get a contradiction with quantum mechanics. So you should give up the ontological models framework, not not Leibnizan principle. So to my mind, those are the good no go theorems because they teach us something about

why this framework is failing. So that's kind of the the key bit, which is like, why do I study these hidden variable models that I don't believe in? Because I want to know precisely where they fail. If if I have wrong ideas about the the failings of the ontological models framework. Like if I have some no go theorems that are just

you know, uh that the logic is wrong or their assumptions are unnatural, then that will be a poor guide to sort of being a revolutionary and replacing this framework with something better. I need to know what is it about the phenomenological quantum theory that really resists explanation? So so that's why I studied. Okay. Uh But okay, so the the coming to PBR now. So the PBR theorem uh is is a a theorem that says we make certain assumptions.

at uh so th I I can get into what they are but t together with the idea that the quantum state is epistemic Derive a contradiction with quantum predictions. And so the conclusion that is drawn is that uh we we don't have a psy epistemic ontological model, we we have to be have a scientific ontological model.

So the so the first thing I say in response to this is I kind of don't care because I am not a proponent of sciepistemic ontological models. I I want to get rid of the ontological models framework. And so, you know, a a a theorem that says you can't have that. I already knew that. Bell's theorem already tells me I can't have uh my Leibniz principle even if I take Psi to be epistemic, right? So Koshenspecker and Bell have already taught me this.

Um but nonetheless, you know, in the in the spirit of It's good to have no go theorems to to guide the way. You can ask, is is the PBR theorem something that w is likely to teach us how to go beyond the ontological models framework? Uh is it likely to be something that'll help me in my research program?

And so that's where my kind of criticism of the details of of the argument come in. Okay, so so th we have to get into the the details of what what is the PPR theorem. So it makes two key assumptions. Um I I'm gonna sort of give a two two passes through what those assumptions are. Uh well you'll you'll see why I'm gonna go through it twice. But the the the first assumption in the popular understanding is what I will call ontic separability.

So it's the idea that if I have a composite system A and B, and I ask what is the ontic state space of that composite? So an ontic state is just a description of all the properties that the system might have.

Um, what is the ontic state space of the composite? Uh, in terms of the ontic state spaces of the components? And the answer is it's it's just The Cartesian product, which means if I want to tell you everything about the properties of the composite system, it's sufficient for me to tell you all the properties of A and all the properties about B, and that's it.

So in other words, it's kind of a commitment to there being no holistic properties. It's it's a reductionist model where the composite it all of its properties come from the properties of its components. Okay. So that's quantic separability. Uh and then the second assumption. I like to call like independence preservation, right? So I have

At the operational level, I might come up to system A and do a preparation procedure on it and come out to B and do some independent preparation procedure. So I imagine I can vary how I prepare A independently of how I'm preparing B.

And in quantum theory, we we model those situations by quantum states that factorize, right? So there's a density operator for A and a density operator for B, and I can vary them independently. And the idea of independence preservation is that in one of these ontological models,

these quantum states are represented by probability distributions that factorize across these two ontic state spaces. So in other words, there's no correlations between the properties of A and B when the preparation is of this type. Okay, so I think that those are very reasonable assumptions. And in particular this this independence uh preservation property

actually follows from Leibnizian principles, right? So I can I can give an argument that says, look, if if um If if it weren't the case, if there were correlations between the properties of A and B, even though as I modify the preparation on A, no measurement I do on B can tell that change. Well that would be a failure of Leibniz because it would be

a ontological distinction. I could learn something about the ontic states of B by, you know, ch changing or learning about A, but there would be no empirical consequences to those differences. So that that would be compromising Leibniz. So So if you know, my commitment to Leibniz says yes, independence preservation is is a great assumption.

And ontic separability also strikes me as a totally natural assumption. And it's true that you can take the uh structure of the PBR argument and and show that those assumptions, together with Psi epistemicity, right? The fact that implies a contradiction. So I have no problem with that. The problem I see is that um it also rules out psionic models. So the way I've just phrased those assumptions.

I can say that cyantic ontological models are also inconsistent with the quantum predictions. And I can do it very trivially, because if I look at an entangled state And I ask, does it satisfy The principle i is is the principle of ontic separability satisfied? If I think quantum states are real and I grant that there's entangled states in the world, no, that an entangled quantum state cannot be understood as being just, you know, a a product quantum state.

It is generally understood by proponents of the scientific view as invoking some holistic properties of the pair of particles. So if ontic separability is saying no holistic properties, well, you know, you you can't be psionic and hold on to ontic separability.

So the way I think of it is ontic separability is the assumption, then here's the easy no-go theorem. You can't have a psy-ontic ontological model. That's trivial. It just follows from the existence of entangled state. And then there's a hard no-go theorem, which is the one that uses the PBR construction.

But at the end of the day, if these are your assumptions, then what you show is that just all ontological models, whether they're scientific or cyepistemic, cannot do justice to the quantum predictions. And so I like to think of that as the the rehabilitated PBR theorem. It's it's like Bell, it's like Coschen Specker. And in my view, the right uh uh lesson is that you have to give up on the framework of ontological models.

Okay. But in fact the the the assumption that they make in the papers is not on acceptability precisely. Uh i I think it is what most people think they're assuming. Most people who look at the PBR argument I think believe that what they're assuming is what I called anti acceptability.

Um but in fact what they need to assume to get their conclusion, right? So what they're trying to show is that psy epistemic ontological models are ruled out, but psyontic ontological models are not ruled out. They're still ruled in.

To get that, you need to make a much more refined, nuanced assumption, which is that I allow holistic properties in the support of distributions that represent entangled states, and I only disallow holistic properties in the supports of distributions that correspond to product home states.

Okay, so so that's the assumption they need to get their conclusion. I I call that the entanglement holism link, right? So when you have an entangled state, then there are holistic properties uh in that certain Now The the the thing is that it most people look at that refined, so it's usually not clear that that's the assumption, but even if that becomes clear, I think most people look at that and say, Yeah, that that's something I believe. So that's that's all good. Uh but in fact

It's only plausible if you already believe that the quantum state is a state of reality. Right? So if you think that a quantum state is ontic, Then you think that an entangled state is a strange way of being. It's a strange way for A and B to exist. It's the strange properties of A and B and there's some sort of connection between them and s some sort of holistic properties.

But with these psi epistemic ontological models that I've worked on, the way they represent a tank state, it's just a correlated probability distribution over Ontic separable state spaces. So so every entangled state just means, oh, if I update my knowledge about A, I'll learn something about B, but there are no holistic properties anywhere. So from the perspective of a psi epistemic ontological model, somebody who comes from that position.

And you ask them, do you want to take on board this assumption that's natural for your opponents? The answer is no, it's it's not natural for a cyepistemic perspective. So it makes the argument kind of circular to say, oh, well, if you grant that this is natural, which you'll only do if you already believe the quantum state is real.

Well then we have an argument that establishes that the quantum state's um right. So st my challenge to those who want to use the PBR theorem as an argument in favor of scientific ontological models is you know, please provide a motivation for this entanglement holism link. That doesn't just rely on your scientific intuitions. You know, it isn't just a reaffirmation of your beliefs that an entangled state is a funny way of being that includes holistic properties.

Can you express, can you motivate it in terms that will be appealing to somebody who believes in a psi-epistemic ontological model to begin with? Where an entangled state is just a way of knowing about a pair of properties, a pair of systems. Um and I that's the the challenge that uh hasn't been met. Um so yeah, my you know, the the the main response I have is sort of like it's it's about ontological models and I don't think we need to keep these.

But I also don't think it's like Bell's theorem of Corsan Specker, insofar as it it's not making natural assumptions. and and ruling out only cyepistemic models. The the natural assumptions, if you make them, rules out both cyepistemic and psyontic models. And so in that regard, it's it's like Bell, it's like Koshenspecker, it's it's another particular proof. that we're gonna have to go beyond the ontological models framework.

So my understanding is in twenty fourteen or so Leefer if I'm pronouncing that correctly as I only read it. Lifer or Leefer Right. Great. So that these Psy epistemic models are exponentially not good at explaining distinguishability. So how do you respond to that? Well yeah, so uh again it's uh a question of like the assumptions that go into these theorems, are they natural assumptions? Um so there's there's many no go theorems that

try to make an assumption about a particular way of explaining the indistinguishability of certain you know non-orthogonal quantum states, for example. And Th there are some assumptions. So so there's a particular assumption called maximal cyepistemicity that that says that, you know, when when two distributions are indistinguishable, it should be entirely explained by kind of the the magnitude of their overlap. So

Kind of you if you tell me how much overlap two distributions have, then sort of classical probability theory will impose a bound on how well you could possibly discriminate them. Uh and and I think that is a reasonable assumption. precisely because you can derive it from the Leibniz principle. It's it's an instance of non contextuality. And so for me, the if if you think that's why it's a good assumption, then really you should assume the deeper principle, the Leibniz's principle.

And there's no way to salvage a scientific model if you assume that principle. We know we can rule it out with Bell's theorem and the Cotian Spectre theorem. So the game becomes, can you give me any reason for believing this assumption about how to discriminate states? that isn't just an appeal to Leibniz, because if it's an appeal to Leibniz, then, you know, uh I I can't salvage scientific ontological

So again, the challenge is like, right, you've got this assumption. Please give me a motivation for this assumption that doesn't just boil down to appealing to Leibniz's principle. Um So I mean there's there's a there's a lot of details I've I've left out, but that's essentially uh the the the state of things.

You know, I i I th th here's here's a kind of subtle point, which is that it's often thought that if if you derive a no go theorem from logically weaker assumptions that somehow that's better. Like, okay, two Nugo theorems, you know, one has Some assumption X and here's another one. And it uses assumption Y that's logically weaker than X. And and so the claim would be like, okay, well that that's somehow impressive.

But but I think that's just a mistake. And and the example I use is that um You know, s suppose assumption X is, you know Uh Newton's laws hold. And assumption Y is Newton's laws hold on Tuesdays and I'm not gonna tell you anything about whether it holds or not the rest of the days of the week. Well that's logically weaker. It c commits me to less.

It's not physically more plausible. If anything, it's physically less plausible. So the its logical weakness has not added to its physical plausibility at all. If you see what I mean. Yes, interesting. Um and so so the mere fact that you can set down an assumption that's implied by Leibniz's principle, but doesn't imply Leibniz's principle.

Is not a reason for thinking that assumption is more physically plausible. It's merely logically weaker. That doesn't make it more physically plausible. Uh the success of Leibniz's principle and the work of Einstein is what makes it a good principle. And if you can derive some particular assumption like local causality or non contextuality from it, those are plausible as well.

Uh but but if you have something and you want to assume only that consequence but not the deeper principle, I'm gonna ask, well, motivate that for me, please. Uh, is that like Newton's laws on Tuesdays with no commitment for the rest of the week?'Cause if so, I don't see the physical plausibility. It's only logically weaker and that's not an argument in favor of it.

So in your mind, in your in your framework, your your model, whatever you want to call it, if it's psi epistemic, if the wave function is representing a knowledge, our knowledge about what? Yeah, that's a a good question. That's the the right question to be asking. Um and the unfortunately the the situation is that, you know, th this is a research program and I would say we don't have a good answer to that question. Uh

But that doesn't mean that, you know, we we don't have evidence that a quantum state might be epistemic. So so my argument is that you can find facts about the phenomenology of quantum states. that lends evidence to them being epistemic without necessarily being able to answer the question of

exactly how do I formulate quantum theory in a way that sort of unscrambles what's reality and what's knowledge of reality and and then tell me exactly what that reality is. I mean I I have ideas about that, but they're they're speculative. So so that's the kind of tricky thing. Like is does that even make sense to say that you can argue for the kind of status of something without having a concrete model of of of what it means? And so let me let me try to explain that.

Please by by an analogy. So um So so my claim about, you know, uh uh why does quantum theory appear so mysterious, you know, for so long? I think it's mostly due to a category mistake, and that category mistake is thinking that a quantum state describes reality when in fact it describes knowledge of reality. So the analogy I want to draw is to like my favorite example of a category mistake, prior category mistake.

uh that that kind of held back progress. And this is the example of the decipherment of Egyptian hieroglyphs. Yes, I I've heard this. I like it. Yeah. Okay. Please. So so hieroglyphs You know, the the last person who could, you know, write and and read hieroglyphs probably died around five hundred AD. And and then, you know, fourteen hundred years later, people were were trying to decipher it, you know, trying to say, well what what do these inscriptions mean?

Um and it was, you know, long forgotten how how to make sense of it. And the prevailing idea uh at the time was that each glyph was an ideogram, which means it represents a concept directly, not through the medium of some spoken language. Okay, the the an alternative was is that the glyph is phonetic. It represents

the sound in some spoken language. So it might represent a vowel or a consonant or a syllable. Um but the prevailing idea was that it's an ideogram. So it's like a sign at at the airport where no matter what language you speak, it'll be clear what this this means. Um and so people were trying to uh decipher hieroglyphs under that ideographic uh idea of what kind of category a a glyph was. And and people wrote books about what all these inscriptions actually said.

under various interpretations. Um and in retrospect, you know, so Champollion, uh uh famously deciphered hieroglyphs, uh one of the big uh moments was sort of the Rosetta stone being discovered. I think that was seventeen ninety nine. Uh and so a few years later we got the decipherment. And in retrospect, it was phonetic, right? The glyphs were phonetic. So so the

ideographic interpretation was just a category mistake and and all the kind of interpretations ended up just being nothing to do with what the real meaning was. Um it's quite surprising that it's phonetic. It looks like it No? Do you? Well I mean S surprising maybe because like we we use an alphabet where, you know, none of the letters look like anything anymore.

Uh but if you know, if you ever do those puzzles where it's like, you know, a picture of a bee and a picture of a leaf and it spells out belief, you know, like we we could do

uh writing with, you know, symbols that represent sounds as well. It's just that, you know, in the early days of writing, because the Egyptian hieroglyphs is one of the very first writing systems. that was kind of the natural way of representing sounds. And over time they evolved into things that were more abstract.

So from a modern perspective, it's a bit odd, but I'm a time saving person. There's there's so much energy that has to go into creating an image to represent a B sound or a Lee or that that's just it's just I I I'd rather just not write.

Yeah. Yeah. I'll just communicate with my voice. Yeah, I I think it's uh we're accustomed to being able to write quickly, but that was an innovation that happened, you know, thousands of years after the invention of writing systems. So In any case, the uh i if you look at the the the story of decipherment, um

You know what, I I I I'm trying to remember where I'm coming back to what point again. What was your question?'Cause I Well, I wanna know what is it about uh what is the side epistemic uh if it's epistemic, it's about what? Yes. And then you were going there. Right. Thank you. Um Good. Okay. So yeah, the the the analogy is gonna try to answer that question. So so so the way the story goes, um

After the discovery of the Rosetta Stone, right? So the Rosetta Stone had an inscription that was in sort of three different scripts. Um Which we knew how to read. What was it? Demotic, which was a a particular version of the hieroglyphs. Uh and so we we knew what the inscription said because we knew how to read the Greek. And so we knew what the hieroglyphs were saying, we just sort of didn't know how they were saying it.

I I like that because it's sort of analogous to how I think about quantum theory, where it's like, okay, I I might know what it's predicting, but what I really want to understand is how how is it making those predictions, right? Like different interpretations will give different stories about how the formulas of quantum mechanics makes the predictions about relative frequency.

Okay, so so people at the time had the Rosetta Stone. They had they had a few other examples of uh Egyptian hieroglyphs in in multiple scripts. Um and a key thing was that Thomas Young, the same guy famous for the young double slit experiment, realized, so he he knew that the Greek script on the Rosetta Stone um had the names of pharaohs in it.

And so the Egyptian hieroglyphs had certain symbols that they were called cartouches, they were kind of circled, and he thought, Well, maybe those are the names of the pharaohs, because that's important. And then he realized there was an assignment of No phonetics, two different symbols that would reproduce the names of the pharaohs that appeared there.

And then people went and looked at other scripts and found that they could reproduce the names of other pharaohs, you know, older pharaohs from times before the the Rosetta stone, using this phonetic interpretation that Young came up with. Um okay, so the strange thing is that people didn't say, oh, okay, so it's all phonetic.

They continued to try to interpret everything in terms of ideograms, and they had an explanation for why prop you know proper names are special. They they would have to be sounded up phonetically. There's no way you could communicate them with concept. And it was only when Champagnon finally said, okay, well let let me try. He was frustrated, and he said, Let me try interpreting all of it phonetically. Um

See what happens. And if if the language spoken by the Egyptians had died out and there was no trace of it. in Champollion's time, he he wouldn't have been able to make progress'cause, you know, he he would be able to s you know, maybe f sound out things, but he it wouldn't correspond to any language that we knew.

But fortunately that hadn't happened. So so the Egyptians uh the the kind of common language spoken by the Egyptians uh at the time did die out as sort of a language that that people used on a regular basis a after the Arabic invasion and Arab became the language they spoke, but it survived in the Christian Coptic Church. So there was a liturgical tradition and and that language

was basically the language of uh uh the ancient Egyptians. And so it just so happened that Champagne had studied Coptic. He knew the language Coptic. And so when he tried this phonetic, interpretation, he started seeing kind of the the pronouns and the grammatical structures of Coptic, which he knew. And so he that was his Eureka moment. It's like, okay, this is a phonetic interpretation and it's Coptic.

Okay. So what's the analogy here? So so what's the analogy here? So so I see the fact that, you know, we have these sub theories of quantum theory, what I call the Clifford subtories, that can admit of a they can they can abide by the Leibniz principle using just a conventional framework for realism.

uh that's like these cartouches, right? So the the the phonetic versus uh ideographic is kind of like the psy epistemic versus sciontic. And we can have a a perfectly local non contextual understanding of a part of quantum theory. Uh and I look at that and say, okay, that's good evidence that all of it is gonna have to be interpreted in a way where the quantum state is epistemic. But to answer the question, you know, knowledge about what, is a bit like knowing the Coptic language.

Right. So so the nice thing about the the names of the pharaohs is that they sounded the same whether it was spoken by a Greek person or an Egyptian person. So they were sort of common. You didn't need to know the language spoken by uh the Egyptian scribes. to interpret the cartouches. Um, but you you did need to know Coptic to be able to decipher the rest. Um, and so I we're kind of in that situation that I I think

You know, uh in the decipherment of hieroglyphs, uh there was you know good reason for people to try phonetic everywhere and then figure out, okay, what's the language relative to which this phonetic interpretation works? And we're kind of there. We need to sort of find uh a way of formulating facts about causation and inference, a a new formalism that's uh a modification from the classical one, much like relativistic space and time are different from non-relativistic space and time.

uh pre-relativistic space and time. So we we need an innovation to the notions of causation and inference. And that will give us sort of the answer to the question, what's this knowledge knowledge uh about? Uh but we're not there yet. So so, you know, we we need to find our Coptic. Uh the the the the thing that I think is also lacking on the side of people who want to defend the view that quantum states are states of reality.

i is that I've never heard a good answer to the question, why is it if it's true that quantum states are states of reality, why is it that the subtories of quantum theory admit of these very natural interpretations, interpretations that preserve Leibniz's principle. but wherein the quantum state is epistemic. You you can only preserve Leibniz's principle if you make the quantum state epistemic. So it's it's a bit like how prior to Champagneau people did have a reason for saying

why it could be that the cartouches were phonetic but everything else was ideographic. They said, Well, it's it's it's the special status of proper name. You know, proper names would naturally have to be sounded out phonetically, unlike other words in the language. And I feel like it would be nice if s if you know, if if we could have if if somebody wants to persuade me that I'm wrong, what they really need to do is say, Well, why is it that

The psi-epistemic point of view works so well in these sub-theories of quantum theory. Why is that just an illusion? Why have you been misled by that? You know, here's the explanation. Um so anyways, that's my attempt to answer, you know, what what it means, or like w uh how to think about a research program where you know that a quantum state is knowledge of something, but you haven't quite figured out what it is.

So we both gotta get going, and I only have a couple of questions left. Something I wanna know. Tell me about Some concept, something you were trying to understand for months, maybe even years, you kept banging your head against the wall, and then all of a sudden it clicked. Whether it was mathematical, it could be philosophy related, it could be anything. I just want to know about that whole process and what was it that made it click. Hmm.

Um I could I can certainly give you an example where maybe I don't go into the details. But more kind of like a sociological Example. Um so back in 2005 or so, I I wrote a paper about extending the notion of non-contextuality to preparations. And so usually it's just talking about measurements. So it's extending it to preparations and and measurements that are not projective and to transformations.

Uh and then a few years later, uh I I showed how you could do an experimental test of this notion, uh which I called generalized non-contextuality. But but though those experimental tests only leverage non contextuality as it applied to preparations. Um and they didn't leverage it for for measurement.

And I couldn't see how to do it. Um and and that's sort of how uh it stood for for many years. I I sort of just didn't quite know how to do an experiment where I would I would make use of this assumption for measurements. Um And so, yeah, maybe it was ten years later or something, I uh uh there was a a student, Ravi Kunjwal, who was uh visiting perimeter, and I said, I think this would be a good problem to come back to. And so I went to the board to explain what the problem was.

Uh and when you are forced to explain something and and what the sort of obstacles to progress are. you often, you know, articulate your ideas more clearly than than, you know, you you would if you didn't need to explain to somebody. So I the amazing thing was that as I found myself trying to communicate properly what the problem was, I immediately saw the solution. Uh it turned out that all all that really needed to be done to to kind of get through the impasse.

was to explain clearly to somebody what the problem was. Uh and that's happened several times in my life as well. Yeah. Even with personal problems. Just well, what what's frustrating you? And then I say it and then there I say the solution as I'm speaking it. Yeah. Yeah, it was it was it probably the the clearest example of that because as as I was sort of I got to a certain point in my explanation, I was like

Ah, there isn't really a research problem here because I d I've now seen, you know, this is what I've explained to you is actually there there's the resolution. And uh so there's some details to be worked out, but but the big question is answered now. Um, so we we can move on. Uh What's the most inspirational advice you've received? That is a good question.

I th I think as I was saying earlier Lee Smolin w in the early days of permanent institute would would really communicate very forcefully the kind of importance of trying to revolutionize physics. Which kind of you know sounds Uh like a lot of hubrists, like oh who am I to revolutionize physics?

But but in the end when you think about it, it's like, well, yes, that is the goal you should set yourself. Um it's it's not overly ambitious. It's not arrogant to set yourself that goal. Uh so I I I think Lee's advice Probably some of the more inspirational advice I've ever gotten, which is like: set your sights high, have audacious goals, try to revolutionize physics. What is it about Lee that's different? I could be more specific.

Many of the physicists that I s most of the physicists I speak to on this channel are the sort that are thinking in terms of how can they contribute strongly to the field of physics. Yep. But many of the physicists that I speak to off air, they they almost disdain this attitude of revolutionizing. One because as you mentioned, there's hubris behind it.

Or maybe it's because of something else. They they've tried and and they've failed and they're the elephant that has tried and failed and so don't try anymore. I forget the the whole parable, but It could be that, it could just be it's too difficult of a problem, it's it could be it doesn't need a revolution, it needs a slight modification, it could be that hey, which is gen which is actually true, almost all the

Knowledge production in science comes from incremental change anyhow. So d don't discount the non-revolutionary, because the non-revolutionary is the is the 95% important part.

So there must be something different about how Lee thinks, as you just mentioned, it was one of the most inspirational pieces of advice. So it doesn't come from many people. It came from Lee. What is it about him? Yeah, I think I think yeah, Lee Lee probably is distinctive in that he values much more highly than than most physicists the

very innovative approaches. So so like new ideas, new approaches, you know um He he understands that most of them are probably wrong, so he sort of describes it as a high risk, high reward kind of situations or like an investor will invest in a lot of high risk. So I think he sees physics as we we need to invest in people who have

very different ideas of how things are gonna go and and some of those are gonna be right and revolutionary. And so he's very supportive of researchers who go their own way and and try something completely different. Um So he's yeah, v very tolerant of that in a way. So like if if you're more conservative and and less and more risk averse, you might say, Well

A little bit of that's fine, but you know, let's let's also stick to kind of what it is c is is is guaranteed to make progress. But yeah, Ali is somehow is uh happy for people to be doing their own thing. Uh, he's happy for, you know, lots of mistakes to be made on on the way to progress. So I think that's why he um yeah, that that's what's different about the kind of advice he gives that he

He certainly w wants you to pursue your own particular path. Um and he sees that as the way that the community can make faster progress. I think to to some extent that that's certainly true. Uh It is difficult. I mean, the sociol sociological reality is that you try something completely new, it might be a very long road before you've made progress. And so how do you get a career going when you haven't got much to show for yourself? So everybody kind of needs to build their career. They can't be

taking too long to make any progress at all. So there it kinda has to be a balance. You have to sort of show that you're capable uh in in ways that don't require you to have sort of finished your research program. Um, but I definitely think that, you know, more tolerance for original ideas is needed. And certainly at Perimeter, our strategy for hiring postdocs over the years has very much been preferential uh hiring of of people who really do have original distinctive research programs.

we we don't hire them into a group to be mentored or supervised by somebody, but rather as independent researchers. And that's a good recipe for, you know, generating some really novel and innovative results. Thank you for spending so much time with me. It's been a pleasure. Now my last question, as you it's a quick one, an easy one.

There's an oracle behind you or whatever. There's an oracle. And you can ask it any one question about quantum mechanics. What do you ask it? And it's obviously it's a truthful oracle. Yeah, okay. Um I'm gonna interpret that as follows that let's let's say the Oracle knows the correct realist interpretation of quantum theory and uh I I'm gonna get a clue from it that will help me in my research program.

Um so I think what I would really want to know is I I I feel strongly that it is possible to unscramble the omelet of causation and inference in the quantum formalism. So I kind of know how to do that classically where the causal stuff is basically you you you could formalize it using category theory. You can say okay the the the objects, the causal relata are set.

And and causal dependences are functions. So I have sort of a category of sets and functions. And then on the side of inference, I have a category of substochastic matrices, which basically represent conditional probability distributions and and these categories interact and and so

when I believe that certain things are causally connected, then I can sort of propagate what I believe about one to, you know, updating my knowledge about the other. And I have a kind of whole formalism for how you disentangle these notions of causation and inference. And we're trying to build that formalism in the quantum case.

Um, but there's a lot of things that I don't know how to do. So I think, you know, the the most valuable clue to me right now would be, all right, just just tell me how to unscramble causation and inference. I I think if I knew how to unscramble causation and inference, you know, in in the mathematical formalism.

I could then make, you know, progress towards, okay, what what is the principle that distinguishes classical theories from quantum theories? And I could ask uh, you know, the next question. Um, but but there's this formal Uh, thing that we haven't sorted out yet, which I feel would be a a great uh help moving forward. Thank you, Rob.

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