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5| QBism: The World is Unfinished — Ruediger Schack
362 Plays1 year ago
Is the fate of the universe predetermined? Many physicists and philosophers argue it is, particularly those who adopt the Many Worlds interpretation of quantum mechanics.
Our guest this week is Ruediger Schack. With Christopher Fuchs and Carlton Caves, he is one of the originators of a new way of interpreting quantum mechanics, QBism, according to which we — as agents — are co-creators of the world. Destiny is shaped by our hands.
Ruediger is a professor of mathematics at Royal Holloway, University of London, and works on problems in quantum information and quantum cryptography, but also seeks to understand what the equations say about the world.
One of the central claims of QBism is that the wavefunction is a representation of knowledge, not physical reality, as such the “collapse of the wavefunction” due to agent interactions is nothing more than Bayesian updating: observations lead us to update our knowledge.
We unpack the ideas of QBism — that reality is not objective, but inter-subjective, using ideas from phenomenology best summarised in Merleau-Ponty’s comment “there is no world without a being in the world”. We also dive into some of the objections to QBism.
This was a foray into foreign waters for me, I hope you enjoy it as much as I did.
Notes:
- More detail on multiverses.xyz
- Ruediger's article on QBism in The Conversation
- Ruediger's paper on Merleau-Ponty and QBism
- Chris Timpson's talk on QBism on YouTube
Recommended
Transcript
Introduction to Quantum Interpretations
00:00:00
Speaker
What does quantum mechanics tell us about the world? This is a question that we've already asked on this podcast, and you might say, I have the answer. David Wallace told us it's the Everett interpretation. We live in a branching multiverse. But today we're going to present a very different view on things. Our guest is Rudiger Schack. Rudiger is a professor of mathematics at the Royal Holloway University of London.
00:00:23
Speaker
He spends his time working at the coal face of quantum mechanics on problems in quantum information and quantum cryptography, but he's also, along with Carlton Caves and Chris Fuchs, one of the originators of a very different way of interpreting quantum mechanics, cubism.
Measurement Problem and Epistemic Views
00:00:41
Speaker
You might recall from the discussion with David Wallace that the key to understanding what quantum mechanics tells us is the measurement problem, the apparent collapse of the wave function when measurements are taken.
00:00:54
Speaker
Now, in common with David, Rudiger believes there is no measurement problem, but he believes that for very different reasons. David thinks that everything happens. Rudiger thinks that actually when a measurement is taken, there is a collapse in the wave function, but that's not a problem because the wave function doesn't represent an underlying state of reality. It just represents the state of your knowledge. It's an epistemic object. So when you take a measurement, of course your knowledge updates.
00:01:24
Speaker
that's discontinuous but it's not mysterious. What I really enjoy about this conversation is that it's a very fresh take on understanding quantum mechanics and it also draws in insights from phenomenology and continental philosophy from people like Merleau-Ponty. Now for someone trained in the analytic tradition such as myself this is unfamiliar territory and it's really fun to explore.
00:01:50
Speaker
And that's what I hope to get from this podcast, ways of exploring different worlds and different conceptions of the world. I'm James Robinson. This is Multiverses. Ruda Gashak, thank you so much for joining me on Multiverses. It's a pleasure.
Rethinking Quantum Mechanics: The Case for Cubism
00:02:22
Speaker
So about, I think around 20 years ago, you embarked on a program with some others to create a new interpretation of quantum mechanics, a new way of understanding what it's telling us. So that would have been about 80 years after quantum mechanics first emerged as a theory, and 80 years over which it's been incredibly, predictably successful.
00:02:47
Speaker
So my first question is, what is it about quantum mechanics that, despite all that predictive success, motivated you to think, well, we still need to do some thinking to understand what it is that quantum mechanics tells us. What's difficult about quantum mechanics in that sense? Well, that's a big question. First of all,
00:03:11
Speaker
You almost mentioned the enormous predictive success and we all agree that quantum mechanics is the most successful theory that physics ever has come up with or has created. On the other hand, these now 100 years have seen discussions on the meaning of quantum mechanics which have never stopped and in some ways never made much progress.
00:03:38
Speaker
Well, on the one hand, quantum mechanics is enormous relative power. On the other hand, there's still the same discussions about what it means about how to interpret quantum mechanics that, well, we had 50 years ago, 80 years ago, et cetera. So that is one background. There's something more specific. Myself, and that's only
00:04:07
Speaker
But it's partially true also for Chris Fox. It's true for Carl Caves and myself, because the cubism arose from this very fruitful early discussion, discussions between Carlton Caves, Christopher Fox and myself in Albuquerque. We were thinking about the meaning of probability and interpretation of probability and where
00:04:36
Speaker
very dissatisfied with the with frequencies or or let's say objective interpretations of probability because they
00:04:50
Speaker
Well, they get you stuck in statistical mechanics. So the interpretation of probability was for me the first, well, the first step towards thinking about the interpretation of quantum mechanics.
Probability and Agent-Based Measurement
00:05:01
Speaker
Now, we all know that quantum mechanics probability is a
00:05:09
Speaker
well, quantum mechanics is a probabilistic theory. Probability plays a central role in quantum mechanics. And I think I believe what Ed Jain said, that it's, I'm probably just paraphrasing him, there's no hope understanding quantum mechanics before you actually understand probability.
00:05:33
Speaker
And so my thinking ability was what we started first, I'm thinking about quantum mechanics in this way. Okay, interesting. And I guess to kind of give a little summary for listeners who might not have caught, there's a previous episode with David Wallace, where we go, into some more detail on the kind of fundamental difficulties at the heart of quantum mechanics. But as you allude to, they're around probability and maybe more specifically,
00:06:03
Speaker
you kind of have two laws. If you open kind of an elementary textbook on quantum mechanics, you might find something saying, OK, you've got two kind of equations of motion, if you like. One is a very familiar sort of wave equation, which is saying everything's evolving quite smoothly. You have some states that are
00:06:28
Speaker
obeying this wave equation. Very nice. And then there's another rule, which is kind of where the probability comes in, which says, oh, when you do something, when you try and measure something, you need to take your states from your first equation and apply something called the Born rule, which kind of collapses them down. And I guess it's that, you know, it's that
00:06:52
Speaker
almost dichotomy or the fact that we have two rules, one which looks to physicists quite familiar in that it's a wave equation, and one that looks quite unfamiliar in that it seems to demand that the observer is playing an important role. And when they are taking a measurement, suddenly there's a new rule that's applied.
00:07:15
Speaker
it seems that this is where Cubism has something quite interesting to say. So, in short, what does the Cubism say about, what is your kind of the program view on what's going on here? Well, if you look at this dichotomy of these two elements of the quantum formalism, then Cubism are things that exactly the opposite
00:07:38
Speaker
point of the spectrum for many worlds where in many worlds the main focus is on the wave equation on the what's called the unitary evolution or the the the law of evolution of the state vector and in quantum in in cubism we de-emphasize this very strongly this is actually not an important part
00:07:59
Speaker
is certainly not the starting point. It's important in many ways, but it's not fundamentally, foundationally important, the deterministic time evolution of the wave factor. Instead, the starting point for cubism is the
00:08:22
Speaker
is measurement. So just that part, which in standard formulations would be associated with something like collapse of the wave function, which is not the term that we actually use. So it's measurement, that's the starting point. Of course, you then immediately would ask, well, what is measurement?
00:08:47
Speaker
And you also mentioned the observer. Now, Cubists, you will notice, never speak of the observer. We actually refer to talk about agents instead. That is important because observer, so we still talk about measurement, right? In a measurement, if you talk about an observer that suggests that the measurement, that the observer just observes something that already exists,
00:09:18
Speaker
And measurement very often, I mean, if you think of measuring the length of an object in everyday language, you of course think, well, your object has a length and I'm just measuring it and determine it. And an observer also reinforces this picture of measurement as discovering, finding something that already exists. In cubism, a measurement is always an action.
00:09:44
Speaker
It's an action that an agent, let's say, it is a quantum measurement, it's a physicist taking an action on some system. The agent participates in the bringing about a measurement outcome. So it's not discovering a pre-existing measurement outcome, but the act of measurement participates in the creation of the outcome.
00:10:10
Speaker
what the quantum formalism gives us, now just a nutshell of cubism, just about the quantum formalism, the quantum formalism, quantum mechanics, the rules of quantum mechanics, so there's a guide to action. They guide the agent in their actions. Brilliant. And so I want to pick up firstly on something you mentioned right at the beginning there, which is that
00:10:36
Speaker
Yeah, the emphasis for kind of many worlds in interpretation is on the wave function and, you know, that's unmysterious, whereas this perceived collapse, and I agree, I don't think there is a collapse, is the thing that needs careful thought. Whereas on the curious perspective, well, this perceived collapse is entirely to be
00:11:02
Speaker
understood as a function of just our Bayesian updating, I think, right? So once you are, you know, when you are preparing your experiments,
00:11:15
Speaker
you're anticipating that you're going to find something. You don't know what it is. And of course, it makes sense that when you find it, there's a kind of discontinuous movement in your beliefs. And I think that's the key thing, obviously, for Cubist that the movement, the change that's happening is not a change in the world. It's not a change
00:11:34
Speaker
in a objective fact about the world. There's no collapse of some object or wave function. Rather, there's an updating of our beliefs, if I understand it correctly. And I think that's quite... What's interesting to me is, in some ways,
00:11:55
Speaker
both the many worlds interpretation and the cubists want to say, there is no measurement problem, right? They say that for very, very different reasons. But I think there's an agreement that there's not an issue there with measurement. The many worlds go a sort of hyper realist route where they say, well, the wave function is so real that all the different things that it's telling us do occur. Whereas the cubists say, almost the opposite, the wave function is not
00:12:25
Speaker
an element of reality at all. And reality is somehow co-created, I guess, between the agent and the quantum system. But it's not a pre-existing thing that we can read off about the world. Is that a fair kind of reinterpretation or summary?
00:12:47
Speaker
Yeah, it's not bad. I think, I think that's pretty, pretty accurate.
Cubism's Philosophical Foundations and Realism
00:12:51
Speaker
So, um, Cleve did your homework. Thank you for that. Thank you. The, yeah, I was, I was said that was a pretty good characterization, also actually different between the two approaches. The, um, but, so what Cubism isn't doing, okay. What you mentioned the, well, the,
00:13:15
Speaker
predictive power or the success of quantum mechanics as a predictive tool, Cubis would say, as a guide to action, which is, well, for us is an important difference. But in any case, it gives us, it allows us to manipulate the world to also to make predictions.
00:13:42
Speaker
to build devices, to do experiments, all kinds, and then in a way that was never done before. The step that what you call realist, well, hyper realist or whatever, what most universal quantum mechanics do, they actually take the mathematical
00:14:03
Speaker
objects, formulas, the object of quantum mechanics of the theory and well, I would say then they mistake them for the reality.
00:14:15
Speaker
If you see what people are actually doing when they're using quantum mechanics, well, you would say they're using this formalism for prediction. I would say they're using it as a guide for action. But anybody who uses it and people who use it are the ones who actually turned it into this very, very successful thing. The success is measured by what you can do with it.
00:14:37
Speaker
no way is it necessary or even I think suggested that these mathematical constructions you have they are that they correspond to something in the real world. So that is an additional step you're taking. You're saying you have a very successful theory, it has all kinds of abstract objects, it manipulates, so these must be somewhat correspond to real things out there.
00:15:03
Speaker
Sometimes this is called reification, reification of things to use. Now, this is something that many worlds is doing and something the cubism isn't doing. Yeah, I think that's a really important distinction to get out there. I first want to say that
00:15:26
Speaker
I think it seems to be a common misconception of Cubism that it is not realist at all, but I think it's just not realist about quantum states in particular. And as you say, the kind of laws that we have under the Cubist perspective, I've seen them often described as normative.
00:15:50
Speaker
rather than being laws of nature that are describing some underlying reality, they are laws that we should seek to follow if we want to sort of get on well with the world as it were. So that their guides to action in that they're normative. I guess my, where I struggle here, because I feel that all makes sense. But where I struggle is,
00:16:20
Speaker
The realist picture in general gives a good reason for why we should follow those laws. They say, well, they're not normative. They're actually reading off some facts about nature.
00:16:39
Speaker
we could sort of make this normative movement for any theory of physics. We could take Newtonian physics of billiard balls and say, well, actually, you know, Newtonian mechanics is not telling us something about the real world. It's just telling us how we should set our beliefs about what's going to happen in
00:17:03
Speaker
games of snooker and in planetary motions, but it's not actually telling us that there are forces out there and so forth. And then I think my move there and my move with quantum mechanics is to say, well, look, if these are giving us such a good guide to action,
00:17:28
Speaker
what is the reason for that? And maybe the reason is that there's something about that guide that is catching or capturing elements of reality. I mean, this is just, I've restated in very long format, you know, inference to the best exploration or abduction, right? And that, you know, if you have a theory that's working really well in some predictive sense,
00:17:52
Speaker
maybe the best explanation of that is that it's actually capturing something real about the world how big question how how does cubism think about that that's an interesting question here's one take on it so you how do we how do we
00:18:19
Speaker
I mean, how do we know? Why do we describe the world in terms of forces and particles, et cetera? Ultimately, because that works extremely well. So we have done experiments and we have physics is an experimental science. So all the theories, the
00:18:45
Speaker
the terms we use to do physics like forces, particles, mass, etc., we all use them because they work extremely well. And now you're asking, now you're trying to explain
00:19:04
Speaker
why they work well, well from, it seems to be circular to me, right? Because you postulate or you actually come up with these theoretical constructs because they work well and then you say, now I understand why they work well because this is how the work is. So I think that it's somewhat circular and doesn't give you much. Cubism on the other hand is more direct. We actually, we have, so to actually say something
00:19:35
Speaker
more clearly that you also mentioned the cubism believes that the the the rules of quantum mechanics tell us something about reality so the world has a property which makes this these particular rules a good normative principle a good a good guide for action so often cubists say this is some some
00:20:04
Speaker
This is the kind of realism that Cubism aims for. It's saying something about the world and the rules of quantum mechanics understood, normatively understood as answering the question, how should I act? They actually are, if you want, a real feature of the real world.
00:20:33
Speaker
So, and asking, well, so in that sense, I believe the answer Cubism gives to why does a work is more direct because the world has a feature which, which says that if you, if you follow these rules, you will be successful and it's objective. So I don't know.
00:20:57
Speaker
whether I want to necessarily call this a form of realism, it's certainly objective rule in the sense that everybody can use it. So unlike a quantum state, which should be going to get to this point, a quantum state or probability in humanism is always one agent's personal subjective belief. The rules of quantum mechanics are objective in that any agent can use them.
00:21:27
Speaker
In that sense, they say something about the world. I'm slightly cautious here only for the following reasons. So, Qism understands itself as a realist project in the sense that it wants to actually learn something about the world. It wants to find out what the world is like.
00:21:47
Speaker
That is, I believe, much harder to do than if you are a standard realist where the world just consists of certain objects. You have a third-person view of them coming up with an ontology that takes the agent where the agent, the subject and the object both are central.
00:22:17
Speaker
is hard. And there are some philosophical that, so there are philosophical or areas of philosophy with that, try to answer the same question, there's pragmatism, but particularly there's phenomenology. And recently we have been in a very fruitful dialogue with phenomenologists. Now phenomenologists would actually not say
00:22:45
Speaker
would not describe the quantum formalism as a feature of reality because they are even more radical. They want to just start the entire project from building it up from the phenomena so that something like the quantum formalism would come later and would not be described in their view as a real, as a pre-existing part of reality. So this is why I qualify this slightly.
00:23:15
Speaker
but I certainly would say that the kind of formalism is objective and because it's normative it gives us a very direct answer of why it works because that is an objective rule which says something about the world which tells us directly what to do and so therefore
00:23:44
Speaker
of my say long-winded answer to that question. No, I think I'm understanding the position and maybe to contrast it with the kind of more standardly realist picture, which would be something like you have these rules and then we need to look sort of further than them for the objects
00:24:12
Speaker
that they, you know, seem to suggest exist. And those are the objects that we're going to place our faith in as kind of members of reality. And it seems, and I might be mischaracterizing here, but that the cubist is saying something like, no, we have the rules. The rules are real. The rules are objective. We can all obey them.
00:24:34
Speaker
But don't try to look further than that. And maybe that's why you say it's a much more direct explanation. Does that ring true or?
00:24:48
Speaker
And I know, by the way, yeah. No, here's the first time I actually really, really disagree with you. I would not say don't look further. The whole project of formal... maybe human has three big, big projects. One is interpret quantum mechanics. And we've done that. We actually, we understand and we can say exactly what every term of quantum mechanics means and how it's used. So it's interpretation of quantum mechanics.
00:25:15
Speaker
The second bit is derive quantum mechanics from simple principles. And that would give us actually more and more insight for why it works, etc. So that's an ongoing project and a lot of progress has been made there and is being made, but it's certainly not complete. And then there is the biggest project, Go Beyond.
00:25:41
Speaker
go beyond just economist and tell us what it tells us about the world. So we want to understand what the world is like. And actually, we have very clear ideas of what that should look like.
00:25:57
Speaker
In that respect, philosophically, going beyond the quantum mechanics, the biggest difference between quantum mechanics and, sorry, between cubism and most other interpretations of quantum mechanics is that, well, cubism describes a, well, it's often called a participatory view of the world. Agents participate in creating the universe.
00:26:26
Speaker
which implies, and this is an important part of it, the universe is unfinished. It's not just there. It does not afford an outside view. There's no God's eye view of the world. There's no third-person view of the world because it's an unfinished thing. And in that sense, it's, I think, a much more
00:26:50
Speaker
Well, it's a much better view of the world. It's a much more interesting view of the world. I regard something like many worlds as rather sterile. You've got, I mean, think of it. You have, what's the ontology? What is ultimately the world? It's a state vector, which is, well, sits in an abstract robot space. And it evolves according to a deterministic equation. So it's essentially,
00:27:18
Speaker
It's all there. Once you have a snapshot of the thing at one time, you can actually follow it backwards and forwards, and the entire universe is just sitting there in front of you. It's a static view of the universe. Whereas cubism points to a, well, to a view of the world that is an unfinished world, a world where every system has autonomy.
00:27:47
Speaker
So by, by, see, we have it here, here's, here's, here's another shot at the, at the fallacy that comes from identifying the objects of your theory was the whole thing was everything with, with, with, with the world. So, because I just before I only
00:28:12
Speaker
mentioned one aspect of this, which is that you have these mathematical constructs, and they're very successful, and so you think, oh, they must correspond to things in the world. You can actually go one step further and say, oh, it's so successful, therefore, they must be all there is in the world.
00:28:30
Speaker
And he was, I think, quite capable of that. They say, so successful, therefore, all the world is this big wave function. Whereas, Cubism says, well, there is actually no evidence for that at all. And Cubism says, whenever we use quantum mechanics to model or to make predictions,
00:28:59
Speaker
or to act on a quantum system, we only ever capture certain very well-discovered aspects of that. It's never the whole thing. We're not saying the wave function we write down for a system just in some sense captures everything about the system. And we will say that every system we approach in this way, we apply quantum mechanics to, has autonomy. So this is where the state we ascribe to the system
00:29:28
Speaker
Well, it's not the ascribe to the system. It's not the description of the system. It just captures our expectations of what we would experience when we act on the system. It leaves the system its autonomy.
00:29:46
Speaker
So the world is, I think, a far more interesting, a far more open and dynamic place than any third-person view can capture according to cubists. So all this was an answer to your suggestion that cubism wants to stop there at the level of making predictions. No, we don't want to stop here. We want to go far beyond that.
00:30:12
Speaker
Good, good. Okay, so I'm glad that it's more ambitious than I yeah, and I think I had that flavor. So maybe I was being a little bit provocative there. And I mean, you mentioned a lot of interesting things that I want to pick on. I first just want to say, I guess the, you know, on the sterility of the many worlds, I mean, certainly it's completely deterministic under the many worlds, or the universe under the many worlds interpretation is completely deterministic.
00:30:41
Speaker
And I guess just very briefly, their response would be something like, well, all the interesting things like agency, you know, are real, you know, features of the world in as much as they're very, you know, you know, powerful patterns of explaining things. And that kind of gives them legitimacy in your, in your theories. But they're, they're emergent. And I think that's maybe an interesting
00:31:07
Speaker
again, point of difference here, where perhaps what the cubist is saying is, well, actually it's fundamental to our way of understanding things, that if you look at quantum mechanics, the agent is really, you know, does feel like an important element of that. And it's not one of these weird mathematical elements that just, we can't figure out how to,
00:31:35
Speaker
sit within reality. But it's naturally telling us that agents are playing an important role again in the kind of co-creation or in this participatory realist picture of things. And therefore, we need to give agents a kind of not just emergent status, but like a kind of fundamental
00:31:57
Speaker
on-text status to use a bit term of art. Is that, so is agenthood, I guess, one of the things in the cubist picture that is playing an explanatory role? Or is an agent just another emergent feature of the world? Or is it not such a simple dichotomy? I think it's neither. In the sense, it is simple. So quantum mechanics,
00:32:27
Speaker
is, and let's say physics in general, from a qubits perspective is, as we mentioned before, guide to action. And so it is a, well, as a guide to action for agents. So that means that within quantum mechanics, agents, well,
00:32:57
Speaker
Yes, yes, something. So it's, in a sense, it's an addition to decision theory, quantum mechanics. It's not describing the world. So it's not a theory that
00:33:14
Speaker
describes how agents emerge, for instance, if you wanted to ask this question. You can ask questions like that, but they always have a different flavor. They're always, so quantum mechanics would always give you, well, you can apply it to other agents. Well, obviously, because quantum mechanics can be applied to any system, but you always need to
00:33:44
Speaker
I need to remember that when I apply quantum mechanics to some other agent or so that all I'm doing is I'm just using the formalism to quantify and organize my expectations upon acting on that system of the other agent.
Phenomenology's Influence on Cubism
00:34:12
Speaker
So that's what the, that's what the role of quantum mechanics is. And therefore it's quite limited in that way. Asking, asking to define agent within quantum mechanics is in that, in that view akin to asking, well, if somebody uses probability theory or classical decision theory, well, before that goes off the ground, first you need to define agents within probability theory, user of the theory. So they're just different categories.
00:34:41
Speaker
the user of the theory. Sorry. Maybe I'm doing you a favor. I have the same myself often. I wish that sometimes I would be interrupted as my thoughts trail off. I guess, yeah, I think that
00:35:07
Speaker
As you say, you can apply. So under the cubist perspective, you can actually apply, you know, quantum mechanics to agents, but I guess you can never completely remove all agents from the picture because there's always someone doing the quantum mechanics as it were. And that's, I guess, key to this. Maybe that's one of the things as you were pointing to with phenomenology that is falling out where you're saying, okay, here.
00:35:35
Speaker
the structure necessarily is, or at least under this, which we think is our best interpretation of quantum mechanics, the structure of the world is such that there is no objective in the term of God's eye view of reality. There's always our most fundamental theory, our best theory at least,
00:36:03
Speaker
of the world is one which I don't know if it demands agents, but a lot of the sort of interesting experimental results actually, if you want to apply that theory, which we do, it demands agents and it demands at least one agent who's preparing experiments.
00:36:25
Speaker
And I'm not sure, this is probably why you need to interrupt me. I'm happy to interrupt you. There is, again, quite a number of things which I might say here. In phenomenology, one of the important, what I mean as a starting point is that there is no
00:36:55
Speaker
There's no way to understand a subject without the world, without the object, no way to understand the object without the subject. There are always different aspects of the same underlying fundamental thing. I think no matter how you then describe this any further, they cannot be separated. So there's no third person
00:37:24
Speaker
view of the world, but there's also no independence, so the agent is also never just in itself without the world. There are two poles, two aspects of the same underlying thing. So when you, at least twice, probably twice mentioned the idea that there should be at least one agent, and that is
00:37:50
Speaker
The idea of just there being one agent is very far from Cubism. Cubism, the world is populated from, well, lots of agents. That sounds very much like a third-person view, but it isn't. There's some very nice way in which Merleau-Ponty brings in
00:38:13
Speaker
other agent is almost necessity of defining yourself. He has this view, has this thought experiment or the observation that you cannot see the back of your own head. But clearly, you have a view of yourself as a person.
00:38:39
Speaker
Now, how can you have that? And so he says that in order to constitute yourself as an agent, as a subject, you actually need the perspective of other subjects. It's another way of saying that the subject cannot be thought independently of the world. It goes further and says a subject can actually not think itself independently of other subjects.
00:39:09
Speaker
And in that way, you have multiple, going back to the language of Cubism, multiple agents, almost as a necessity.
00:39:22
Speaker
In cubism, there's another way of expressing this, there's this big, we always stress that other systems, any other parts of the world have autonomy. So when we write down a quantum state for another system, well, repeating that phrase, this only captures my expectations for the consequences of my future actions.
00:39:51
Speaker
it does not constrain the other system in any way. It's not the quantum states together with the laws of quantum mechanics don't tell the other system what to do. The other system has fundamental autonomy. Yeah, yeah, yeah. That makes sense. So, you know, there is no, it's not meaningful, at least in the world that we live in, where we acknowledge there's lots of agents, there's no kind of
00:40:19
Speaker
It's very anti-idealist, or anti-solipsist, I guess, in that there's no sort of one person who's calling the shots here, right? It's co-created by everyone. And I have a couple of nice quotes from Merleau-Ponty, and thanks for sort of pointing me to him, which I thought I'd just throw in here as some kind of... I feel like they...
00:40:42
Speaker
echo what you were saying. So one thing that he said was there is no world without a human being or without human beings in the world. And I feel that captures this idea that the phenomenologists and also the cubists are neither objective realists in the sense of there being the possibility of a world without
00:41:12
Speaker
without some kind of agent, I guess, or rather the world is co-created with kind of external features plus agents together. But it also captures the fact that, you know, there is some kind of external reality. It's not just idealism. There is, I guess in Merleau-Ponty's terms, there is this kind of flash, which is linking things. And maybe in Cuba's terms, that's
00:41:42
Speaker
the actions of Asians. The other quote, I'm just going to see if I can find it here that I have from him, written down many things. I think, and this is maybe more controversial and maybe where phenomenology and cubism diverge, but Merleau-Ponty at some point said that
00:42:08
Speaker
I think it's in the phenomenology of perception. He wrote that phenomenology is a matter of describing, not of explaining or analyzing. And he kind of clarified, I think, his meaning a bit there by saying, it's a method of returning to the thing themselves, to things in themselves. I guess rather than imposing kind of further structure around those things to explain them.
00:42:38
Speaker
I mean, I think you've already stated that cubism does seek to be explanatory, but perhaps there's an element of what Merleau-Ponty is saying here that it's trying to, you know, the phenomenology is trying to describe things in themselves. Does that feel in the spirit of cubism, if not the idea of langicide?
00:43:07
Speaker
the ambition of explanation. Well, I'm not an expert in phenomenology and so it's a bit, I'm not, I'm a little,
00:43:24
Speaker
some, some hesitation comments more about this, but here's one, one, one thing in, in any phenomenal, phenomenal, phenomenal logic reconstruction of the world. So quantum mechanics comes very late. It's, it's, it's,
00:43:48
Speaker
So cubism only starts quantum mechanics only starts to, to, um, to, to appear when there is an agent and actually it's converse has to be a physicist has been very well educated agent, um, who has identified a system. So there's, there's already a very clear split between the subject and the object. The, the, the physicist will have to have identified a physical system that,
00:44:17
Speaker
to make an experiment on, to query in some way, write on a quantum state. So this all comes very late in the phenomenological constitution of the world or so. So in that sense, I must be careful not to try to compare these enterprises too closely. Phenomenology
00:44:43
Speaker
Cubism is in some sense more ambitious or maybe has a different starting point. Phenomenology is pure philosophy.
00:45:01
Speaker
observe your, your, your, your experiences, there is a thing called epoch a where, where you just try to get rid of all your preconceptions and start and start from with a with just your, your basic, your, your unanalysed experiences. So cubism, we believe that
00:45:30
Speaker
our view of the world, our conception of reality of the world, is really motivated, is almost compelled by quantum mechanics itself. So it's sort of empirical in that way. It comes from the particular features of the quantum formalism.
00:45:51
Speaker
And phenomenologists would not say this normally. So we think that there are reasons, if you look at the formula, if you look at quantum mechanics, you can see why we make certain certain choices and you and you're compelled to a certain view of the world. And so that's the that's the hubris project. So it is, it is physics more than just it is, it is
00:46:16
Speaker
it is physics driving philosophy more than the other around, even though as there's of course there should be really conversions of ideas, etc. So it's not just one direction, but physics plays a major role in that whole enterprise. Yeah, that makes sense. So yeah, the motivations are different, but they sort of somewhat converge, you know, not completely overlapping, I think, on a similar view of
00:46:45
Speaker
of the world, which is one that goes, I guess, against the grain of thousands of years of the traditional way of understanding things of an objective reality. And I feel like one needs very strong motivations to give up that picture.
Cubism vs. Traditional Quantum Explanations
00:47:11
Speaker
I want to come back to some of the
00:47:16
Speaker
maybe let's sharpen up this intuition a little bit. So one of the things I wanted to touch on was the EPR criterion of reality. So in the sort of the famous Einstein-Rosenbodolsky paper where they talk about entanglement, they put down a criterion, which is something like this. If you can predict with absolute certainty
00:47:44
Speaker
the outcome of, or the value of some certain variable, I'm paraphrasing here, then there's some element of reality which kind of corresponds to that variable. And I guess kind of rephrasing that in, you know, kind of more simple terms, you know, if you know, I think this comes back actually to this say, original point around
00:48:13
Speaker
the simplest way of explaining why laws work seems to often be that there's some straightforward way in which those laws are capturing elements of reality. And I think that's the intuition behind the EPI criterion, right? If you can for sure say that something's gonna be, you know, that some pointer is gonna point at a certain value, then, you know, whatever theory is, you know, whatever that value is, it's kind of representing something real about reality.
00:48:43
Speaker
And then, of course, the way that this gets carried through is, well, you can set things up so that you can know with certainty the values of a quantum state. And if that's the case, well, isn't the quantum state itself something real? And I know there's a... Yeah, so how does a cubist kind of think about that?
00:49:13
Speaker
Qism certainly rejects the EPR criterion of reality. One reason is that the EPR criterion of reality leads to big trouble in quantum mechanics, as we know. It leads to this idea of non-locality, which is
00:49:31
Speaker
very, I mean, this science fiction sounding idea that you can make a little, you can change something here instantaneously, let's say on Sirius, or some far away star, something, something real changes. It's, that's a direct consequence of the EPR Criterion of Reality plus quantum mechanics. Yeah, so it's got a nonlocality, which is, which is, for instance, goes against anything that Einstein believed about the world.
00:50:04
Speaker
There is a much simpler way of actually seeing what's wrong with this. It's a category mistake. If I believe something of a certainty,
00:50:18
Speaker
Why does it mean there's something? I mean, I could just be wrong. It's just my belief. And my belief does not, has no hold on the world. So for a cubist, there is no difference between, no fundamental difference between writing down a probability one half or something and probability one. I just, it just expresses my belief. Probabilities do that. And it's, and that seems a very, very,
00:50:48
Speaker
obvious poem that I must not never even I mean even the community of physicists can be wrong about something even if they believe it with with them with certainty probability one is does not mean there's an element of reality connected to it that's just that's just a category mistake
00:51:09
Speaker
I think that's a strong response. I think that really captures it. One might still feel that there is a justification to have a categorical difference between almost certain of something and just knowing something based on facts.
00:51:35
Speaker
But I'll accept, I mean, I think this is like a rabbit hole we can... It's found this very slightly. So it's a very common way of embedding probability in logic, where true and false correspond to probability 1 and 0.
00:51:52
Speaker
And so regarding probability theory is an extension of logic. Now, in that context, it makes perfect sense to identify probability one with truth and probability zero with falsehood. But in cubism, it's based on an interpretation of probability in terms of action, decision theory, where probabilities
00:52:20
Speaker
the role is to inform action. And, and so all the probability one then says is really says, well, you will, you will bet all your everything you own on an outcome because you're right on probability one for it. So that's all it means. It just it just is a numerical expression of a
00:52:49
Speaker
of, of the kind of action you will undertake. Yeah, good. And before I, I think, I want to discuss some other pieces here, but I think this is like an important, I just want to restate the fact that I guess this locality problem is one of the kind of key things that, you know, Cubists would say, we're successful on that, right? If you want locality,
00:53:16
Speaker
like the many worlds interpretation, that's one of the things we offer. So again, in common with many worlds, you're both saying, okay, there's no, there's no measurement problem. And there's no issue with locality. And again, the reasons for making those claims are completely opposite on the kind of every Aeternan or many worlds view, locality is preserved. And maybe
00:53:45
Speaker
Very briefly, the EPR experiment is, you know, have these two entangled particles. And it seems naively that if one is on one side of the universe and the other is on the other, and you perform an operation on one, it's going to instantaneously affect the state of the other. The many worlds folks will say no, there is a, you know, actually, you know,
00:54:13
Speaker
things have already by this point split into two branches of the wave function, or they're splitting as you run the experiment and you discover that in one world, both particles were already in a kind of aligned set of states, let's say. And in another world, you're just discovering that they were already in a disaligned set. So you're uncovering facts about the worlds that are already in here.
00:54:42
Speaker
And on the cubist perspectives, you're saying, no, there are no facts to be uncovered here about the world. We're just updating our beliefs and, and therefore there's no kind of instantaneous change in, you know, a state of reality that that's caused by. So, um, just to, uh,
00:55:06
Speaker
We phrase this, so the entangled state you're talking about, that is the state of a particle here and a particle in this location, is always the state of one agent. So for instance, if I'm here and I have a particle here and a particle over there, it's my belief about what I would find if I walk over there and make a measurement on the other particle.
00:55:33
Speaker
And so when I make a measurement here, then I only update my beliefs about what they would find if it walked over to the other particular measurement there. So that's how I thought. Yeah. Just gets rid of all that, all that nomenclality nonsense. It is a very nice explanation. I want to come back to another kind of aspect of realism, or rather, I guess, argument for
00:56:01
Speaker
they're being something real to quantum states. And it goes something like this, which is that if you have, under the Schrödinger equation or other equations of quantum mechanics, quantum states evolve through time, even if you're not interacting with them. And the kind of Cupid's perspective on this seems to entail us to believe something like,
00:56:29
Speaker
the following, that we need to be kind of continuously updating our kind of epistemic beliefs, even though we've not received any new information about the world. The only information that we've received is that sort of time has passed. I'm not saying this is a knockdown argument, but I feel that's kind of unintuitive and in that generally
00:56:59
Speaker
I can't think of any other examples where we want to update our beliefs unless either we have received new information about things or we feel that there is some time-driven physical process which would be causing things to change independently of what we're doing.
00:57:22
Speaker
Is this, you know, do you feel the same difficulty here? Is there something that, that, that Cubist Neil need to grapple with, or is there a kind of straightforward way of avoiding it? It is something that Cubist have to, have to address. And we have very clear ideas about, about this, but they are, well, well, they're,
00:57:52
Speaker
I don't have a very good non-technical way of expressing that. But here's an example of a situation where you actually do change your beliefs without any actual, well, new information.
00:58:16
Speaker
So it has to do with a famous, well, insight of the philosopher van Fraassen, his reflection principle. It's about, you are thinking about how you should gamble, let's say, or how you should act at some future. So basically, you are trying to
00:58:46
Speaker
to let's say you're forced now to put down bets on some events at a future moment. And so there are some principles and ideas of how you can actually formalize that in a coherent way, which do not which bypass or
00:59:13
Speaker
well, actually destroy your dichotomy of updating because something really happens in the world or because you actually had an interaction with the world. So you can actually, there's interesting ideas in that direction. It is something that needs attention and needs further work. Yeah, I feel it's, I think Harvey Brown at some point commented that
00:59:43
Speaker
of what's mysterious for most interpretations of quantum mechanics is the apparent collapse. And that's what they grapple with. And on the Cuba side, it seems to be a little bit the reverse, and it's the kind of evolution of things independently of agents' interactions.
01:00:07
Speaker
Yeah, so it's evolution. Cubits would never say the evolution is happening out there in the world. Because the evolution will always be the change of an agent's beliefs. Because the evolution affects state vectors and state vectors are not existing out there in the world. So the evolution happens. It's about the evolution of the expectations of the agent. And could you maybe, I'm not familiar with
01:00:38
Speaker
Sorry, I was going to say, go ahead. What I was going to add is that the way that happens and the way Cubis would actually describe this and answer the question of why should expectations change in this particular way is something we're working on.
01:01:04
Speaker
Okay, good, okay. I'm keen, I don't know if you can sort of restate the reflection principle of Rand Fasson, because I thought that was really interesting as a potential, you know, just to have an example of where one might be kind of justified in updating beliefs without there being some sort of new information coming through. I'm not familiar with the principle, but
01:01:32
Speaker
Did I understand it to be something like one is reflecting, you know, one just thinks hard about things and therefore ends up changing one's strategy? Is that how it works with betting or is it? What you do is you are now you consider
01:01:52
Speaker
receiving more additional information. Sorry, you are considering now receiving additional information or, or, or doing an experiment in the future. And so, and that means since you're planning to do this experiment in the future, you already now change your, your, your, your betting strategies for, for this, for
01:02:22
Speaker
well, for events that happen even further in the future. So you consider this intermediate experiment, let's say, and simply by considering this by planning, considering this intermediate experiment in some way, maybe gathering more information, you will already change your
01:02:53
Speaker
commitments now. I think reflecting back from the future time to your present expectations and beliefs. I think that makes sense.
01:03:15
Speaker
It's certainly more cashed out than just one thing's hard. It's one thinking. There are very good examples of that in option pricing or so, where you asked yourself, how much should I pay now for an option that
01:03:33
Speaker
that and for some stock option and it depends on the expectations of the various things that might happen in the future. So that is pretty exactly an example of Van Fassen's reflection, option pricing. Okay, that's interesting.
01:04:01
Speaker
I'm sort of wondering whether I have sort of more arguments of a similar vein as to, and I'd refer listeners to, Harvey Brown's got a talk on, where he talks about sort of plausibility arguments for the realism of the wave function. I think he's actually quite
01:04:21
Speaker
I think it goes on the record as saying none of these are like knockdown arguments like that. They're just reasons why there's some explanatory weight to or appears to be some useful work that the wave function is doing for us. I might just kind of drop them out in case you want to sort of fire them down, although I think we're probably not going to do
01:04:45
Speaker
of the objections or the responses complete justice in the time we have. But there's things of the type, well, interference we can understand via the wave function. And then it gives us quite a nice explanation for things like the two slit experiments and just thinking in terms of objects that feel familiar, waves. Another nice one I think is, and maybe I'll finish with this, is around the
01:05:16
Speaker
Exclusion principle and that so for fermions over half integer spins particles you have obviously They can't occupy You know two identical particles can't occupy the same quantum state and there's a kind of explanation by the wave function which goes well Fermions are defined by if you if you swap them between two different states there's a
01:05:42
Speaker
the wave function changes sign and therefore you can't have two in the same state because you'd have one in the same kind of real state of affairs having both a positive and negative sign because you could kind of swap those two and you know that would kind of point to well I mean I don't again I think there's probably other ways that one can draft the exclusion principle or possibly explain it
01:06:12
Speaker
But yeah, curious to get sort of like the, you know, the brief cubist response to objections of this type. Well, I have a very, very simple response, which is the explanation you just gave were in terms of wave functions. Now, if a cubist agent, if a cubist agent writes down these wave functions, then exactly the same explanation will be available to him. The cubist
01:06:42
Speaker
the way function expresses the hubris belief about the system. And then as a consequence of that, you find the exclusion because the exclusion principle comes, is basically a property of the way function you write down. Okay, so I guess the... I mean, explanation has worked slightly different in cubism. Yes, a cubist would say the following.
01:07:07
Speaker
People obviously demand explanation. Cubism says especially we don't explain. It's just a guide to action. But you could actually argue the following.
01:07:18
Speaker
So imagine I have a web of beliefs, which obviously is not just writing something down and pulling it out of a hat, but it comes from very hard work and taking, when you have to learn physics, you have to really work very hard to get a consistent mesh of beliefs because the normative rule is very demanding. It tells you you have to be consistent. And there's lots of information, lots of measurement results, lots of theoretical
01:07:49
Speaker
considerations, and you have to work very hard to make them all into a consistent measure of belief. From all this come the wave functions you assign to particular systems. And now you predict something to happen with probability close to one. And then the typical challenge to Cubans is, so here you have something, how do you explain this? And the Cubans will answer, what do you mean?
01:08:19
Speaker
Why should I be surprised? I predict that this is happening with almost certainty. So what more explanation do I need? I would be surprised if it didn't happen. Then I would have to think back and look at my assumptions, etc. But if all this big normative enterprise of coming up with a consistent picture leads me to a probability close to one and then the thing happens, there's nothing better to explain.
01:08:42
Speaker
Yeah, I think that's like what this comes down to is maybe what is it that we expect our explanations to do? And if we're expecting them to sort of remove surprise, then yeah, clearly that cubism works, right? Because you wouldn't be surprised in that sense. But I think that sometimes we're demanding something more of them, which is not just sort of telling us what we should expect to see,
01:09:11
Speaker
but actually sort of grounding that in something, yeah, in some elements that are out there. So I think you can kind of, maybe depending on your theory of explanation, it might affect whether you kind of, the attraction for cubism versus other interpretations. Does that sound right? Well, yes, but I also would say that cubism very much wants to ground things into something bigger, into,
01:09:39
Speaker
into a picture of the world. And it's just, in a sense, is more ambitious. We don't want to ground it simply in a mechanism that explains or describes everything that physics has to deal with, sorry, physics deals with, actually, I believe, is only a small part of the world.
01:10:08
Speaker
So that's maybe the biggest difference. For Cubist physics, quantum mechanics physics is a small theory. It has a specific role, it's extremely successful in what it's doing, but it's not everything they say, it's not the whole world.
Cubism's Broader Worldview and Scientific Impact
01:10:26
Speaker
And so Cubism's
01:10:28
Speaker
big project the philosophical project is to say well we physics that's a lot and it's it's it's it's um and what does it tell us about this this bigger world the world beyond physics and the the premises there is that physics is a small theory which is uh of course they're metric opposite to lots of other approaches which hope to
01:10:56
Speaker
well to find, to, to, to, uh, we just hope that physics in some sense, some sense captures the whole and everything else is emerged. Yeah. Yeah. So yeah, I think there's, you know, something very humanistically attractive about cubism in, in that sense that, and I, I kind of goes back to your comments earlier that, you know, the, the universe is, um,
01:11:24
Speaker
is unfinished, right? Whereas in like a multi-verse approach or many other ideas, you can kind of think of it as this great, sadic thing in time, which in some emergent manner is producing, sorry, it's not even in time, it's just outside of time, right? But in some emergent manner is producing within us the perception that things going on. But in another sense, it's already
01:11:52
Speaker
determined and written. I do find that is a kind of attractive viewpoint, certainly. I somewhat worry that its seduction is kind of leading on away from the real, you know, parsimonious theory, which would be, you know, and it's unusual to call many worlds theory parsimonious because it posits a lot of worlds. But in other ways, it is parsimonious in terms of not
01:12:24
Speaker
sort of stepping outside of that physics viewpoint of things and just saying, well, all we have is kind of the normal sort of stuff that we've been thinking about for thousands of years. And we don't need to introduce this new element of where the world and the agent touch. I'm curious, do you have...
01:12:51
Speaker
aside from maybe the kind of palatability of some of the consequences of many worlds, do you have kind of other objections to it or what does it all sort of derive from? Okay, let's just I know that some people find comfort in the idea of the world being deterministic and
01:13:21
Speaker
well, really, quantum mechanics, to be all there is. Now there I can really say much against. One key objection for me is, I mean, there are problems in details in twice of watts.
01:13:41
Speaker
has a number of branches, what exactly triggers branching, all kinds of things, which I do not think have been worked out. There is a big problem for me, and I once contributed to a paper about that, actually understanding and interpreting probability in many worlds. I know that
01:14:05
Speaker
many of those proponents believe that problems has been solved and disagree with that, but we cannot really get into that now. The problem is if everything behaves deterministically and everything happens at once, what's the point of probability? Of course, I know there are attempted answers, but there are also rebuttals to these attempted answers. Then there is
01:14:34
Speaker
I believe what's a total lack of so it's probably asking too much of many roles but it's probably I don't think it's unreasonable so you start with this conception of having a function of the universe and a time evolution of that well that's just
01:14:55
Speaker
Really, if you, it's probably too technically a term, but this is a Hilbert space, it's just a vast dimension of Hilbert space in a single vector, and that sort of rotates a particular way. Now, it seems to me that there is no way, just if you're given that, there's no way of getting down to the actual world we see in all this richness.
01:15:20
Speaker
So I believe that many worlds has to cheat and has to take the world as it's given to us and all its marvelous richness and beauty as a starting point and given, even though it claims that the starting point was really there is this abstracting out there. So I think that's cheating. But that's just my, none of these, of course, knock out arguments or anything.
01:15:47
Speaker
There is one way, we're not going to decide this now, but here is one very good reason for taking Cubism seriously. It exists, it's a possibility.
01:16:02
Speaker
it's empirically adequate. So but just by doing physics, you're not going to be able to because interpretation of quantum mechanics, as long as they are coherent and consistent, make the same predictions. So therefore, there's no way of doing experiments to distinguish between them. Just the very fact that there exists an interpretation of quantum mechanics, cubism, which
01:16:24
Speaker
which the world is unfinished and much richer than what physics alone can tell you about is I think exciting.
01:16:35
Speaker
And as you know, as I said, I believe that there are very strong reasons in the quantum formalism to go towards cubism. So actually, it's more than that. For me, the arguments for cubism are much stronger than just saying, well, it is a possibility which is entirely consistent with everything we know. But
01:16:59
Speaker
But even if you had only that, if you only knew Cubism is a possible impression of quantum mechanics, then that means you should actually really look into it because it paints a world which is very different and actually very exciting, very different from the one that's painted by the same many worlds. Yeah, I think that's it.
01:17:23
Speaker
I'm going to say that there's, for me, a very loose analogy here with kind of Pascal's Wager in that you may not have sort of, I mean, as you said, there's arguments coming from the quantum formalism itself that motivate Cubists to take this perspective. But there's also a separate tug, which is the one for me, somewhat analogous to Pascal's Wager, which is that
01:17:48
Speaker
It just gives us a nicer view of the world in terms of, or one that's maybe more comforting. I don't want to be provocative in the use of that word. I mean, or liberating perhaps is a better word to use. Certainly one which makes agency not just an emergent, rather tricky thing to explain, but a kind of fact within the world.
01:18:19
Speaker
Even though agents, as you say, can be themselves subjected to analysis by other agents, there's something special about agency and it's what makes the world unfinished. I buy that. I like the feeling of that view.
01:18:40
Speaker
even though I feel like, as I said, I'm being seduced away from something which is maybe sort of more realistic. Another point, actually, I just want to get in another point here. I heard Dean Rickles comment, and I don't know how strongly he subscribes to this, or if it was a throwaway sort of comment, that one can kind of view the different interpretations of quantum mechanics as
01:19:05
Speaker
if you like having the status of different coordinate systems within relativity, or let's say kind of different gauges. So as you say, there's no necessarily empirical difference between most of them. Some of them do have, make different empirical claims, but most, most agree on the predictions that you'll get out of quantum mechanics at every level.
01:19:32
Speaker
So there's kind of a question, well, what are we arguing over, right? Of course, I think we both feel that we are arguing over something substantive. And maybe that's the biggest kind of agreement between all the different people trying to interpret this, that it's not just enough to get the predictions. One is the Cubist, the Everettian, you know,
01:19:58
Speaker
even the hidden variables folks, are trying to look for those kind of elements of reality and we're just disagreeing on what they are, those kind of explanatory factors. Yeah, so I don't know how much sympathy you have for this kind of view of things as just, or interpretations just as coordinate changes of reference, but I feel like we agree these are important questions.
01:20:24
Speaker
Yes. No, actually I don't, I don't agree because if you, sorry, I don't agree with the, with Dan Rickles point of these just all being just coordinate transformations and just looking at the same thing from a different angle because the, the, I think one of the big differences between cubism and mostly all the others interpretations or the majority of them is that cubism
01:20:54
Speaker
does not think of quantum mechanics as being an inscription of the world in any way, and also not exhausting the world. It's a small theory, whereas all the others are big theories. They actually try to capture the whole, whereas quantum, whereas cubism, because cubism, quantum mechanics only captures a small, a small thing. So therefore, I don't think that this, this, this, this coordinate transformation view is, is, is correct here. But
01:21:24
Speaker
So, I mean, I think that's another point because we want to learn what the world is like. And so we are trying to explore as much as possible. Well, what worldviews are compatible with quantum mechanics because you are believing quantum mechanics is correct in a or is almost successful theory and, and, uh,
01:21:52
Speaker
So we want to know what does it tell us about the world and to come to different answers there. I think we start at the same point in that quantum mechanics is itself sort of not wearing its interpretation or its meaning on its sleeve. I sometimes think of
01:22:18
Speaker
William Harvey's theory of blood circulation as just like a great example of a theory where the predictions and the claims on reality are sort of so transparent that there's kind of no divergence between the two. Whereas quantum mechanics is this thing where everyone's agreeing on the predictions, but the claims about reality seems to be disagreeing sometimes very acrimoniously as well. But yeah, I think again, yeah, it's important to
01:22:47
Speaker
take seriously this business of trying to figure out actually what is behind that predictive success. I wanted to, I mean, you sort of ahead of being, or prior to taking this role within Cubism and developing that program,
01:23:07
Speaker
you know, are a professor of mathematics and have worked many years on quantum information and quantum cryptography. Has cubism changed your practices as a scientist in that role? Or given them a different flavor? That's a very good question. And actually, this is one thing where I think we can
01:23:36
Speaker
we might be able to distinguish between interpretations in a different way. So an interpretation is leading you towards the kind of questions you might ask. So the mathematical theorems or the mathematical questions concerning quantum mechanics that you work on, or that you find interesting depend on your interpretation. And so you can ask what
01:24:03
Speaker
what interpretation is most fruitful in terms of the questions it suggests and questions which are important. And so in one of the theorems that came out of cubism that I proved together with Chris Fuchs and Carl Cave, the quantum definitive theorem is a very good example of that. So the quantum definitive theorem
01:24:32
Speaker
tries to answer the question, what do we mean when we talk about an unknown quantum state? So if you think of the quantum state as something out there in the world, then it's obvious what you mean. Well, there is a system, what the quantum state is just unknown, we want to find it out. In cubism, that's not available to you because the quantum state is always an agent's bundle of expectations, if you like.
01:25:00
Speaker
and personal to the relation. So what do you mean by it? And so the quantum definitive theorem answers that question in an operational way and shows exactly what assumptions you make in order to
01:25:23
Speaker
so that the mathematics that people use to profile unknown quantum state applies. So it identifies exactly what assumptions are behind the very idea of an unknown quantum state. And so that's, that's a, that's a, so yeah, that's, that's a direct answer to your question. This is something where my, my interest interpretation has directly affected the kind of work I did.
01:25:52
Speaker
And there are the examples of that. It also, I think it certainly is, the Von Devenet theorem also goes further. Science relies very strongly on this notion of repeatable experiment.
01:26:08
Speaker
Any experiment that will tell you that a repeatable experiment is not an obvious primitive because you have to work extremely hard to make sure the conditions are the same and it's actually not even totally fine what that exactly means because more or less a lot of circularity in there. So there's no way that the idea of repeatability is just obvious and you just look at it and you find it.
01:26:33
Speaker
And so the assumptions in the quantum differential theorem are actually, in some sense, you can say that they're very precise mathematical assumptions that characterize what you mean by a repeated experiment. So a bold claim would be that cubism is a bold claim.
01:26:54
Speaker
is, in that sense, more fruitful than other interpretations, because it leads to more interesting mathematical questions. But I'm sure you could, you could, you could read about that in some way. I think the weaker claim, and I doubt many would disagree with this, is that there's value in having a multiplicity of interpretation.
Personal Reflections on Cubism and Human Agency
01:27:14
Speaker
So in the spirit of Dean Rickle's comment, you know, while I think
01:27:20
Speaker
It's important to acknowledge that there are significant differences between these interpretations. That doesn't mean they should be, you know, we should put all our efforts into one of them. And I feel there's probably, you know, there's different research programs being
01:27:38
Speaker
encouraged by the different interpretations. I think the many worlds folks would point to quantum computing itself as something that's quite naturally suggested and I think David Deutsch did this from that interpretation and it gives you a nice handle on what seems to be going on in quantum computers.
01:28:00
Speaker
And I'm sure you can kind of rephrase that and say, well, you know, the Cuba's perspective, we can account for that, too. But it's more that, you know, they suggest different ways of thinking about things and probably provoke one. Yeah, I don't I am a great fan of a paper by I think Anderson with the title, a quantum computer needs only one world. But it's, it's interesting.
01:28:26
Speaker
It's an interesting debate on whether quantum or whether many worlds really, really gives you a better handle on quantum computing. I don't believe so, but I don't think we can get into that here. I think that's right. And I'd say the many worlders would say, well, we might have gone along to looking at some of the problems in mathematics that have been inspired by the cubist approach. Actually, Andrew Steen was one of my lecturers when I was at Oxford.
01:28:56
Speaker
remember him, although we never got to that level of discussion in his lectures. But I wanted to ask one other sort of, so I've asked the question how it's affected you sort of professionally. Bigger question, how has cubism changed your view of yourself and of humans in general?
01:29:27
Speaker
I'm sort of asking you to summarize everything that you said in a sense. I am more certain in my belief that I as a human being are autonomous and free and that I'm a free agent and that I have
01:29:55
Speaker
Well, I have free will or that I am an autonomous agent in the world. And I'm not affected by all the arguments that people constantly
01:30:09
Speaker
the sides and sort of in the press and in articles and in discussions that physics actually tells us no, it's all an illusion. It's in popular culture, it's a very powerful idea. Physics, everything, the whole world evolves according to physical law, therefore, well, free will of human agency is just an illusion.
01:30:34
Speaker
And it's a powerful idea. It's very often quoted. And if you have a traditional view of physics, it's seductive. It's worrying. And I have left this completely behind. I just believe in human agency and the fundamental role for us in the world.
01:30:59
Speaker
I think it's a lovely sentiment. I agree. I think we do have agency, we do have free will, but the route I get there is very different and it's sort of the same sort of route as one would use for explaining, I dunno, why it makes sense to believe in
01:31:30
Speaker
I don't know, sheep or something, given that sheep are just loads of... Physics doesn't say anything about sheep. Why should we believe in them? But I will warrant that it seems that the Cubist picture, you know, again, provokes that or encourages that thought in a much more direct way than other interpretations. Yeah, I can't think... I mean, I think this is a good and positive note to finish on, that
01:32:00
Speaker
You know, we're finishing this podcast, but it could end in many different ways. Like in these last, the universe is not written. We're co-creating it right now. We're participating in realism, but not in a sort of us being in a box sort of way. We're pushing on the walls of it.
01:32:22
Speaker
Yeah, it's been a real pleasure speaking with you, Rudiger, and I know there was some kind of trepidation about meeting someone from the other side of the chasm, as it were, the abyss between interpretations, but I feel like we have a lot in common, right? We agree that this is an important thing to think about. We go very different routes from there, but I found this a really interesting discussion.
01:32:53
Speaker
I thought so too. Thanks a lot for asking good questions and it was a very good experience. Thank you.