# Why QBism is completely empty

A good friend of mine, Jacques Pienaar, has recently converted to QBism, as often happens to people that spend too much time around Chris Fuchs. Saddened by these news, I’ve decided to write a blog post explaining why QBism doesn’t contribute anything to understanding Nature. On the contrary, it is a step backwards, as it doesn’t accept the basic premise that there is a world out there and that our job is to understand it. QBism insists that all the puzzles and paradoxes of quantum mechanics don’t actually matter because it’s all subjective anyway.

The first problem with this attitude, of course, is that it’s solipsism. QBists maintain that quantum states are subjective, merely a degree of belief, and therefore everything that is ultimately a quantum state is also subjective: measurement outcomes, Hamiltonians, unitaries, atoms, molecules, trees, capivaras, other people, planets, galaxies, etc. Not yourself, though. Despite being made of atoms, you somehow avoid this subjectiveness, and are unquestionable real. You are also definitely not a quantum state, and cannot be in a superposition. But isn’t that inconsistent with other people being subjective? No, say the QBists. Quantum theory, they maintain, is a single-user theory. You can use it, and it will work for you. Jacques can also use it, and it will work for him. But you both together? Nope, that’s forbidden. And when your predictions conflict with Jacques’? It doesn’t matter, everything is subjective anyway. Hence, solipsism.

Hopefully I don’t need to spend more than a couple of lines explaining why solipsism is such a terrible idea, because I won’t. I’ll merely remark that solipsism can explain any conceivable experience you might have, being therefore no explanation at all. Also that it is not even possible for two different people to agree that solipsism is a great idea, because they will each have their own, incompatible version of solipsism.

Solipsism has such a bad reputation that QBists bristle at this accusation and reply, offended, that of course they are not solipsists, they accept the existence of an objective reality. When pressed on what this objective reality is, then, if not quantum states, they reply that Hilbert space dimension is definitely real. And… that’s it? We just have some Hilbert spaces with some given dimensions, and we’re supposed to model the world with that? Of course not, they say, clearly Hilbert space dimension is not enough. But they can’t say what else is real, and don’t seem terribly interested in finding out. QBism is nearly two decades old, and still they only go on and on about the subjective stuff. I find this attitude incomprehensible. Surely after you identify which parts of your theory are merely subjective, you would discard them and focus on understanding the objective parts, you know, to understand the world out there?

Ok, if they can’t say what is real, can they at least say how their subjective quantum states relate to reality? Are they a probability distribution over the unknown real stuff, as in a $\psi$-epistemic model? Some sort of hidden-variable theory? No, no, no, they say, of course not, they would then be vulnerable to all sorts of no-go theorems. Their quantum states are just beliefs, and how these beliefs relate to reality can never be specified. See, you can never be wrong if you don’t say anything definite!

Let’s turn our attention to the QBist “explanation” for two quantum puzzles. First, the old classic, the nonlocal collapse of the wave function. The explanation, as you might have guessed, is that it doesn’t matter, because the wave function is just subjective anyway. Well, ok, but what happens in reality then? How are the nonlocal correlations produced? Their answer is “Who knows, who cares, and what is this reality thing anyway? And by the way we’re definitely not solipsists!” Over and over again I hear somebody repeating that this QBist “explanation” works as well as the explanation for the nonlocal collapse of classical correlations. It doesn’t. In the classical case the probability distribution that collapses is indeed subjective, arising from ignorance of the actual state of the system. And we can explain how the correlations arise, because the local hidden-variables theory is actually true! Classical relativistic mechanics is indeed local and deterministic. But to apply this explanation to the quantum regime makes no sense: the local hidden-variables theory is not true. And to simply ignore the necessity of being able to produce the correlations in a local way, as the QBists do, is just ridiculous.

The second puzzle is Wigner’s friend, the gedankenexperiment that’s all the rage in the arXiv nowadays. I have explained it in detail before, but the short version is that Wigner’s friend is stuck inside a decoherence-free box, where she makes a measurement on a qubit that’s in the state $\frac{\ket{0}+\ket{1}}{\sqrt2}$. Assuming that the measurement is unitary, this results in the usual world-splitting represented as
$\ket{\text{Wigner}}\frac{\ket{\text{friend}_0}\ket{0} + \ket{\text{friend}_1}\ket{1}}{\sqrt2}.$Wigner will make a measurement of his friend together with the qubit in the Bell basis, expecting to get result $\Phi^+$ with probability 1. If the friend is a Many-Worlder, both of her copies friend$_0$ and friend$_1$ will have no trouble agreeing with this conclusion, but if she is a QBist both of her copies will believe that her measurement caused a collapse, and therefore that Wigner will get result $\Phi^+$ or $\Phi^-$ with probability $1/2$. It takes a bit of mental gymnastics to get to this result, as QBism forbids her from assigning a quantum state to herself and to consider what Wigner will observe. In any case, her expectations are incompatible with Wigner’s, but that’s ok, as everything is subjective anyway and therefore it is not a problem if different agents assign different probabilities to the same event.

That’s what Rüdiger Schack said in a lecture in Vienna several years ago. Apparently this conclusion was too solipsistic even for the QBists, because they recently released a paper with a different take on this: now they refuse to say which probabilities the friend will assign to Wigner’s measurement, saying that it depends on too many things. They emphasize that the friend is not required to agree with Wigner, and that she can anyway learn what the relevant probabilities are if the experiment is repeated many times.

Now that’s just lame. This is a gedankenexperiment, for the love of Zeus, you’re free to specify whatever you think was not precise enough in order to get an answer. Just refusing to give one is the same trick of never being wrong by never saying anything.

That’s why I much prefer Bohmian mechanics and collapse models to QBism. At least they’re trying to do the right thing, and they’re wrong. QBism is not even trying, and is not even wrong.

EDIT: Shortly after posting this I got an angry email from Chris Fuchs. He sent a link to the post to his group, cc’ed to me, with the comment:

An actual scientist would wait until understanding what he was talking about before speaking. But the author of this surely knows his own inadequacies deep inside. My guess is they eat at him.

I think he needs a hug.

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### 22 Responses to Why QBism is completely empty

1. Jacques Pienaar says:

With friends like you Mateus, who needs enemies? It is ironic that although we both went through the Vienna school of Copenhagen, we came out on opposite sides of the force: you a many-worlder and me a QBist. Very well old friend, let us duel.

But it seems you have come to this battle unarmed — most of your claims about QBism are flatly contradicted by what is written in QBist papers. The only thing you got right was the spelling (with a capital QB, not everyone gets it). You grasp of QBism is so tenuous that rather than reply to your individual points, I’ll have to give a crash course on QBism to set the record straight. Granted, QBism is a weird interpretation by most physicists’ standards, but I hope you’ll see that it’s definitely not solipsism, and it definitely has a unique vision for what quantum theory says about reality.

EXPERIENCES
The basic building blocks of reality in QBism are things like I turned on the laser and saw the red spot’, or I went to the museum and saw a Van Gogh’. In short they are experiences’. (Of course, when doing physics experiments we are only interested in a particular subset of experiences, involving sophisticated instruments). The word experience’ is unfortunate, because when some people hear it they think “oh, experiences happen in the mind”. Their logic goes something like this: “anything that happens must either occur in the world or in my mind. Since the nature of the experience depends on me, it must happen in my mind”. The idea that things must be either in the world or in the mind comes from Descartes, so I’ll call this the Cartesian fallacy. Now repeat after me: my experience depends on me, but it does not happen inside my mind’. Keep repeating till it makes sense. Here’s a tip: think of any simple, commonplace experience, like sitting down on a chair or taking a bite out of a sandwich. On one hand, that experience definitely depends on you: another person will not experience that chair or sandwich in exactly the same way as you do. On the other hand, you don’t get to control what it is like: it depends on how the world is. If you hate pickles and there’s a pickle in your sandwich, you can’t make it go away just by wishing. So, your experience is unique to you, but it is not in your mind. Ok? Here’s another tip: remember the etymology of experience’. It comes from ex-‘, meaning out’, and peritus’, meaning to have gone through’. It is something you get from literally going out and going through something. Or if you’re into philosophy, you could try comparing it to Heidegger’s concept of Dasein. An experience may have aspects’ of a mental or a worldly character, but in itself an experience belongs to neither category. Once you grasp that, we can discuss reality.

REALITY
Take a bunch of experiences. In physics, the experiences are treated kind of like the initial conditions’ in a physics problem. QBism doesn’t tell you what they are: you have to decide what experiences are relevant to whatever situation you’re trying to model. Quantum theory (according to QBism) just takes the experiences you give it as input, and then gives you a recipe to predict the likelihood of possible future experiences. Remember that experiences make up reality in QBism, so quantum theory takes reality as input and gives you predictions about reality as output. Pretty far from solipsism I’d say.

AGENTS
Each experience happens to someone’, who in QBism is called an agent’. Just like the experiences themselves, QBism doesn’t tell you what an agent is — you get to decide whether you want to model a given system as an agent or not, and after you make your choice, QBism tells you what to expect based on your choice. There are some minimal guidelines of course: an agent must be able to have experiences. You should consider yourself as the prototypical example of an agent. Beyond that, it’s flexible: maybe for some purposes it might be useful to model a dog, a bee-swarm, a company, or an AI as an agent. Who knows? Try and see if it works. This flexibility is a strength, not a weakness. It allows you to potentially model a wide range of phenomena.

OBJECTIVITY
Okay, so QBism is all about reality, but it seems like a kind of personal’ reality: each experience is only referred to one agent’. Does this mean two agents cannot have the same experience? If so, wouldn’t that amount to at least a weak kind of solipsism? Not necessarily. Agents can recognize other agents and communicate with them about their experiences. Even if, at a fundamental level, no two agents’ experiences are the same, it is clear that a correspondence of sorts exists between them, and it is this correspondence that gives rise to the objective character’ of the world. You might not have exactly the same experience of my sandwich as me, but we can be sure it will be similar in a few ways. We might say that experiences have an inter-subjective’ aspect to them, and our scientific notions of objectivity are founded upon this inter-subjectivity. Exactly how this comes about is still an open question in QBism. That is why although QBism actually says a lot about reality’, it has so far said very little about objective’ reality. Hey, it is a hard problem, we are working on it.

QUANTUM THEORY
You could think of QBism’s vision of reality as the result of a certain methodological process that is still underway. The process is to take the Ramsey / de Finetti flavour of the subjective Bayesian interpretation of probability, and apply it indiscriminately to every aspect of quantum theory, while at the same time adhering to a version of Einstein’s program of the real’. (Fun fact: many supposed criticisms of QBism are actually directed at the subjective Bayesian approach to probability, and have little or nothing to do with QBism per se).

So far, QBism seems to be saying that the world is some kind of complicated mash-up of experiential reality’ out of which objective things can emerge under the right conditions, but where reality is fundamentally subject-dependent. While it is true that many no-go theorems fail to apply to QBism, this is mainly because QBism’s vision of reality is so radically different that it falls outside the scope of the kinds of ontological models that the no-go theorems are designed to rule out. If we had more people thinking outside the box beyond the standard ontological models framework, someone might actually find a no-go theorem applicable to QBism, which would then tell us something new and interesting.

LOCALITY
Admittedly, over time QBists have said contradictory things about locality. Nowadays we don’t claim that quantum theory is local’, but prefer to say that it is consistent with causality’ (with caveats), or at least that it is not non-local’. The truth is, I think, that locality’ is actually not yet well-defined in QBism, so we simply can’t pose the question yet. I have said the same thing about Rovelli’s relational QM in a paper [https://arxiv.org/abs/1807.06457], but much of what I said there could be applied to QBism as well. In essence, every agent experiences space-time in their own way, so to speak about locality we need to find a way of stitching together’ the different agents’ space-times into something like an objective’ space-time manifold, relative to which locality’ could be defined.

About Wigner’s friend: I do think you have a point when you accuse QBism of not saying enough about Wigner’s friend. QBism can definitely say more interesting things here, but to do so requires making additional assumptions about the priors of Wigner and the Friend and working out their consequences. Simply put, this work hasn’t been done yet. Although I think Eric Cavalcanti makes a good start of it near the end of his recent paper [https://arxiv.org/abs/2008.05100].

2. Devin says:

@Jacques What you write doesn’t really contradict what’s above. The main point I took out of it was that QBism doesn’t say much about objective reality, which you agree with. QBism doesn’t provide any objective model and the idea of an inter-subjective network of experiences is even compatible with many worlds or multi-solipsism.

3. Mateus Araújo says:

Thanks for chiming in, Jacques. Well, at least neither of us came out of Vienna as a Copenhagener!

You haven’t pointed out any contradiction between what I said and the QBists write. I’ll be happy to correct any misrepresentation you can find. I’d just ask a bit more of respect: I do know what I’m talking about, I have read way too many QBist papers. I had in fact a lot of sympathy for QBism until I started reading them more closely.

As for your main point: I’m well aware that QBists claim that personal experience is real. That’s fine. My experience of biting a sandwich consists of electrical currents in my brain, they are perfectly real and objective. The problem is your refusal to abstract away your subjective experience and get to the objective sandwich. Quantum theory is about sandwiches, not human neurology.

And it’s rather disingenuous to focus on our appreciation of sandwiches. If our experiences conflict, it’s a rather harmless conflict that can be resolved by understanding that one likes pickles but not the other. It doesn’t bring into question the very existence of the pickle. The conflicts that QBism refuses to solve, or even acknowledge as valid problems, are of a much deeper nature, of Wigner and the friend assigning different quantum states to the same system. How can you not care about solving the conflict? How is it not solipsist to have different people having their own, incompatible realities?

It’s not enough to merely assert that your theory is not solipsist, you have to actually get out of agents’ heads and describe objective reality. Insofar as QBism talks about subjective experience, it is a solipsist theory. Insofar as it talks about objective reality, well, it’s pretty empty. And after such a long time I cannot take your word for it and have to assume that nothing about objective reality is forthcoming.

As for locality and Wigner’s friend, I don’t see any contradiction between what I wrote and what you wrote.

4. Jacques Pienaar says:

Hi Mateus,

Let me try to clarify where I think your original post got it wrong. Are you saying that:

– You agree that reality consists in experiences, which are not in the minds of the agents, and have to do with an extenal world? If so, then it can’t be solipsism by the definition given in the link you posted.

– You agree that QBism is an ongoing methodological program to identify objective features of experience? And that it proposes at least a couple? If so, then it is not true that QBism doesn’t care about this, makes no effort on this front, or is simply evading it.

Maybe your complaint is just that QBism is too slow in delivering on its promises, which makes you skeptical that they will ever be able to. That’s a fair criticism, but it’s the only one still standing from your original post as far as I can tell.

Here’s my defense of QBism’s slowness. Firstly, its core principles were only articulated around 2009. Since then, very few people have worked on it, it’s received little funding, and most QBists’ energy has been wasted in fruitless debates with people who are too lazy to check the definition of solipsism’. Cut us some slack! Secondly, the project of defining objectivity as an emergent feature of inter-subjective interactions is HARD. Philosophers still argue about it, but I think the finer details are just beginning to be worked out in the philosophy of metrology (mostly by this one guy). QBism has barely had the time to absorb this, let alone incorporate it into our program. Third and finally, what’s the damn hurry? We’re trying get QM right, not convert the masses to our religion by selling them a slick finished product. To quote Schrödinger:

In an honest search for knowledge you quite often have to abide by ignorance for an indefinite period. Instead of filling a gap by guesswork, genuine science prefers to put up with it; and this, not so much from conscientious scruples about telling lies, as from the consideration that, however irksome the gap may be, its obliteration by a fake removes the urge to seek after a tenable answer. So efficiently may attention be diverted that the answer is missed even when, by good luck, it comes close at hand. The steadfastness in standing up to a non liquet, nay in appreciating it as a stimulus and a signpost to further quest, is a natural and indispensable disposition in the mind of a scientist.’

5. Mateus Araújo says:

That’s not misrepresentation, that’s disagreement.

I duly noted that QBists claim not to be solipsists; I’m arguing that the actual contents of the interpretation imply otherwise. You profess belief in an external world, but you only ascribe objective reality to your personal experience, that “has to do with an external world”, but which is otherwise unknowable. That sounds like the belief of those Christians that only go to the church on Easter and Christmas. I don’t see a material difference between that and outright denying its existence, saying that all that exists is one’s personal experience. Also, you don’t even pretend to believe in the objective reality of other agents.

I definitely do not agree that “QBism is an ongoing methodological program to identify objective features of experience”. The problem is not that progress is too slow, the problem is that I don’t see any attempt in this direction. Despite the little funding, there’s a couple of QBist papers every year, and they are all about dismissing aspects of the world as subjective, never about figuring out what the objective reality is. To quote my grandfather: “If you want to find an armadillo you have to search for it”.

Maybe one day QBism will start doing it, then I’ll change my mind. But the QBism that exists today is just a hidden-variables theory without the hidden variables.

6. Danylo says:

After thinking about this objective-epistemic stuff I came to conclusion that the “real” question behind this is how to differentiate “I” (the ego) from the “Environment” (the world around). And I don’t have answer to that – solipsism is obviously wrong, and you are not just your body. You consist of thoughts that are in constant diffusion with other people thoughts. You may claim that a human have a definite (objective) state of mind (and consciousness, whatever this is) in any particular moment. But how we can be sure about this? With all this quantum weirdness…

7. Mateus Araújo says:

There’s another point I want to make about no-go theorems. The lack of a precise definition of reality in QBism makes it impossible for it to be a sensible exercise. If I did come up with a definition of reality myself, and provided a no-go theorem based on it, you could simply dismiss that saying that it was not what you meant. You could accuse me of setting up a straw-man just to set it on fire, and it would indeed be precisely what I would be doing. The definition must come from an actual supporter of the theory.

This is what historically happened with local hidden variables, for example. Einstein gave a definition in the EPR paper that he actually stood behind, and this is what made Bell’s theorem so devastating: it wasn’t attacking a straw-man, but a philosophical position that was actually defended. More recently, the same could be said about $\psi$-epistemic models. Harrigan and Spekkens provided a precise definition, and they in fact deeply cared about them. This did in fact lead to fruitful research and no-go theorems that hurt.

8. Jacques Pienaar says:

“Get out of agents’ heads?” So you think that experience’ happens inside agents’ heads: you’re stuck in a Cartesian worldview. For you, subjective means of the mind’, and objective means of the mind-independent world’. To be fair, this is what most physicists would understand by those words, having been brought up in the Cartesian way of thinking. And on these terms, I think it would be fair to call QBism solipsist. But that would me missing the point, because QBism explicitly says that is not how we should think about experience, objectivity, and subjectivity.

So how do we think about it? Let’s take the example of a chair. Suppose I experience bumping into the chair and hurt myself. A QBist would say that’s mostly subjective, because it only happened to me. But it is also a little bit objective because it involves an object, namely the chair, which is experienced by me as something external to my own consciousness. Now suppose you also bump into it, and 1000 other people bump into it, and we discuss our experiences and establish that we’re all talking about the same chair. Now the chair is a lot more objective and a lot less subjective. So for QBists, subjectivity and objectivity come by degrees, they are not all-or-nothing propositions. And the agents’ experiences just exist, they are not inside a mind’ or inside a world’. If anything, mind and world are to be found inside the experience!

I get that this might sound like nonsense to you, if you’ve only ever known the Cartesian way of thinking. I want to emphasize that QBists didn’t invent this. The idea that experience is more fundamental than the mind-world split has a mature philosophical tradition behind it. For me, the main reference for this way of thinking is the philosophical school of Phenomenology’, which goes back to Husserl from the 1930s onwards. The book Phenomenology: the Basics by Dan Zahavi is very readable and I think is essential for everyone interested in QBism. There’s also a movement in cognitive science called enactivism, whose approach to consciousness is very QBism compatible.

This is also relevant to your comment about no-go theorems and ontological models. The nice thing about Cartesian thinking is that you can cleanly separate the elements of reality’ from measurements’. Then you just represent them by mathematical symbols and come up with a function that combines them to give you something obersvable, and voilà, you have an ontological model! In our case the elements of reality refer to an external world but on the other hand are inextricable from the measurements themselves. Just take a look at Chapter 2 of Zahavi’s book, which deals with the relationship between mind and world, and tell me what the hell would a mathematical model of that look like? It beats me! Lorenzo Catani tells me he has an idea that might work for QBism, using point-free topological spaces’. It might work, but it sure won’t fit into the world of Harrigan and Spekkens.

Sure, I can see how to a skeptic on the outside it might seem like we secretly want to remain vague so that we can never get caught out. But I’m telling you, as an insider, that we’d love to have an ontology that can be attacked and examined by the community. We’ve been having group meetings about QBist ontology since June. It’s just bloody hard to invent one, because this is still quite unfamiliar territory. I’d be willing to bet with you that QBism will endorse at least one proposed ontology by the end of 2022. Is it cheating if I end up proposing it myself?

PS. Where do you get that QBists “don’t even pretend to believe in the objective reality of other agents”? I’m genuinely confused how you got that idea. I can tell you with absolute certainty that every QBist believes in the reality of multiple agents.

9. Mateus Araújo says:

Yeah, I’m stuck in a Cartesian worldview. Also known as using the words objective and subjective in the way they’re usually understood. You’re trying to evade the accusation of solipsism by changing the very meaning of objective. That’s intellectual dishonesty.

I’m skeptical that you can get any coherent ontology using these degrees of objectivity and making experience the fundamental building block of reality. Or perhaps you will, simply by redefing what an ontology is.

In any case, I’m glad to hear that you are actually working on an ontology. When it shows up on the arXiv I’ll change my mind about QBism. Provided, of course, that it is an actual ontology according to my definition. Not that I’m demanding it to fit into the Harrigan&Spekkens framework. That’s just one way to formalise it, and one that already has plenty of no-go theorems against it.

About the reality of other agents: the interpretation is quite explicit that you also assign a quantum state to other agents, and they are as subjective as anything else. Perhaps you mean that you believe in their reality in a informal, pre-theoretic kind of way. I’m talking about how they are actually formalised in the interpretation.

10. Jacques Pienaar says:

Well, I don’t know what you think QBism gains from its supposed ‘intellectual dishonesty’… all it seems to do is make people think we’re solipsists! But I concede that it’s our burden to be clear about what we mean by ‘subjective’ and ‘objective’, and it is hard to find a complete and unambiguous discussion of this point in the literature. That’s because we’re still in the early days of articulating this, and it is tricky business.

On a more technical point: in QBism we usually assume (implicitly) that all agents agree that there is a system, and what its dimension is, even if they disagree about its state. Of course, this assumption can always be questioned within QBism, since at bottom, even the notion of a system of some dimension has to be founded on the experiences of individual agents. Roughly speaking there is a hierarchy of objectivity, depending on how stable a given belief has proven to be against the arrival of new facts.

For example, in a typical lab situation, it is quite likely that something could happen that would cause you to revise the state you assigned to an electron, but comparatively unlikely that something would happen to cause you to believe it is a photon instead of an electron. So in that context we’re entitled to have the prior belief that ‘the system is an electron’ and call it objective. Similarly, when you assign a state to another agent, you might plausibly revise the state you assign to them based on your interactions with them, but nothing they do is likely to make you suddenly realize that they are, in fact, a wooden table. So in this context we can say ‘the fact of an agent’ is about as objective as ‘the fact of a table’ or ‘the fact of an electron’, while at the same time insisting that the ‘state’ of these objects can be largely subjective.

11. Mateus Araújo says:

So not even the Hilbert space dimension is completely objective in QBism? This is getting rather frustrating.

I suppose the point is that you can’t really make a superposition of a photon and an electron, so you can’t formulate the usual puzzles of quantum mechanics by using the photoness versus electroness property. Therefore, you don’t insist that this property must be subjective, as you don’t seem to get into trouble otherwise.

Except, though, you can. Pair production transforms a photon into an electron-positron pair. As it’s fundamentally random, you do get a superposition of the transformation having occurred or not. Or, more prosaically, neutron decay. It will be in a superposition of being a neutron and a proton-electron pair. I suppose then you’d shift your not-quite-subjective system to being the appropriate bosonic or fermionic fields, and the subjective state to being the number of excitations in each field. The Hilbert space dimension should then shift from being the dimension of whatever degree of freedom you were interested in, to being the usual infinite dimension of QFT.

12. Yash Sharma says:

Hello,
About the part where Jacques Pienaar described the reality aspect of QBism –
‘Quantum theory (according to QBism) just takes the experiences you give it as input, and then gives you a recipe to predict the likelihood of possible future experiences.’

The agent instantiates one experience, but what happened to all the other experiences that were also predicted with a certain probability?

Many worlds says that all those remaining experiences happen as well, and collapse theories say through some contrived mechanism that only one happens.

How does QBism do away with all the other experiences?
(I know exceptionally little about physics, so if my questions are completely off the mark please correct.)

13. Mateus Araújo says:

Yash Sharma,

Your question is right on spot. There isn’t a satisfactory answer, though, precisely because of what I’ve been complaining about all this time: QBism refuses to say anything about objective reality.

Since QBists insist that the collapse is only subjective, and the probabilities are only subjective, I think what it would make sense is to say that the experience that was instantiated was pre-determined by a hidden variable, and the probabilities are just ignorance of which value the hidden variable actually took. Just how it is in Bohmian mechanics.

You can be sure, though, that QBists will emphatically say that this is not the case, that theirs is not a hidden-variables theory. But they won’t say what is going on, they won’t go for real collapse, Many-Worlds, or hidden variables.

14. Jacques Pienaar says:

Hi Yash Sharma,

When you ask ‘what happens’ to the unrealized possibilities, it sounds like you are presuming that, in some sense, these possibilities ‘exist’ even before they are measured. But a QBist would say ‘possibile events’ do not exist, so it does not make sense to ask ‘what happens to them’ when some event occurs. When something happens, it is real, but we cannot say that it was real before it happened.

Think of a coin. Before you flip it, you say that there are two possibilities: either it will be heads, or tails. If you believe in an objective interpretation of probability (like Mateus does), then this is a statement about the physical properties of the coin itself: the possibility of heads’ and the possibility of tails’ correspond to something that really exists in the coin.

It might be that you think there is a variable that determines what the outcome will be, which is unknown to you, but which is the only possibility and it exists even before you flip the coin. This would be similar to the hidden variables interpretation. Or it might be that you think the coin has an intrinsic tendency’ to land either heads to tails, and this tendency is an objective physical property of the coin. When you flip it and get, say, heads, then this property physically changes into heads’, and the possibility of tails is destroyed’ in the process. This is similar to an objective collapse’ interpretation. Finally, you might think that tails’ is not destroyed, but simply happens in a parallel part of the multiverse, to an exact copy of you. This would be similar to the ‘many worlds’ interpretation. All of these options treat the ‘possibilities’ as describing something real about the coin.

QBism rejects all of those interpretations. QBism says that before we flip the coin, the possibilities of heads and tails simply do not exist. A ‘possibility’ just means something we think might happen, but it does not correspond to anything actual in the world. The possibility of heads or tails before you flip the coin does not say anything about the coin itself: it only says something about what you think might happen in the future, and nothing more.

When you flip the coin, something just spontaneously happens, and whatever happens is real in the moment that it happens. However, it did not come from something that existed before. It was created in the moment that the coin was observed.

We like to think of it as similar to the Big Bang. In some cosmological models, the Big Bang was not caused by anything that existed previously. For QBism, every time you flip a coin and get a result, you have made a little bang’ (Chris Fuchs calls it a QBoom). This little bang brings something new into reality that never existed before. By flipping the coin and observing the outcome, you have participated in creating a tiny piece of the universe.

Notice that I say ‘participated’. That is because although you played a part in it, you do not get to say what actually happens. That is up to the world. As Anton Zeilinger once said, ‘There are two fundamental freedoms: our freedom to define which measurement apparatus to use and thus to determine which quality can become reality; and Nature’s freedom to give the answer she likes’.

15. Mateus Araújo says:

That’s just avoiding the question. You can’t escape Maudlin’s trilemma: to solve the measurement problem you have to either

1. give up the assumption that the quantum state encodes everything about the relevant physics (e.g. by postulating hidden variables).
2. give up the assumption that the quantum state evolves linearly (e.g. by having a physical collapse).
3. give up the assumption that measurements have a single outcome (i.e. going Many-Worlds).
16. Yash Sharma says:

Hello Jacques Pienaar,
From your explanation i am able to understand a little better how QBism views ‘multiple possibilities’ and that it considers them as not being real. In case of the coin flip example, it is clear.

But how does this view explain situations where interference is actively exploited, such as in the case of Shor’s algorithm. The result and the measurement of the output of the algorithm can be understood in terms of an agent and its experience. But how do we explain with QBism, the underlying reality that makes it possible for a quantum computer to perform those computations?

17. Jacques Pienaar says:

@Mateus: Suppose you keep saying to me in chess you only have two options: either you choose black or white. Now choose.’ And suppose I keep saying to you, but we are playing Scrabble’. Am I avoiding your question, or are you avoiding my answer?

Maudlin assumes that the quantum state is (directly or indirectly) a representation of a system’s physical properties. He takes this for granted and never acknowledges that it is an assumption. We physicists are all raised on this dogma: starting from the measured properties of a system, we are supposed to infer its present ‘state’. We then we apply the laws of physics to we predict the future state, and thereby predict its future measurable properties. Okay, so physics begins and ends with measured properties, and those at least correspond to reality. The ‘states following laws’ simply tells us how to get from presently measured reality to future measurable reality. But does that mean the states themselves represent anything real? There are ways to make the exact same predictions without using the notion of ‘states following laws’ at all.

To put it bluntly, in QBism, the state is not a representation of a system’s real properties — not directly or indirectly, not completely or partially. We don’t need a ‘state’ to be able to talk about reality. QBism is not susceptible Maudlin’s trilemma because it rejects the premises on which it is founded.

18. Mateus Araújo says:

This option falls squarely within Maudlin’s framework. If a quantum state is not a representation of a system’s real properties – either directly, indirectly, completely, or partially – it is in particular not a complete representation of the relevant physics. You’re choosing alternative 1, then.

The only implicit assumption he makes is that there are real properties. If you deny that, sure, you escape his trilemma. But then you’re also a solipsist.

19. gentzen says:

Maybe Maudlin himself formulated his trilemma more carefully, but I have the feeling that the given formulation is either not applicable to some analyses via “carricature decoherence setups”, or else makes a ton of unstated assumptions. Such a carricature uses
three nested systems:

Many attempts to interpret quantum mechanics do so by looking at three nested systems. The largest system is essentially the universe or the environment. The smallest system is the one being observed and following the laws of the theory, and the middle system contains the measurement device or the observer.

So there are at least three different states which must be distinguished:
(1) The “known” quantum state of the smallest system which is observed. This one does have a collapse, so it does not (always) evolve linearly.
(2) The “unknown” quantum state of the middle system containing the smallest system and the measurement device (but probably not the observer), for which at least the “relevant” laws how the “unknown” quantum state evolves should still be postulated. This one has an evolution that should be approximately linear (no longer a collapse, just a weak coupling to an environment).
(3) Both the state and the evolution of the enviroment are unknown. Maybe gravity or something else induces some non-linear evolution, maybe not. However, they are mostly “irrelevant” for predictions concerning the smallest system.

The measurement happens on the measurement device in the middle system. Somehow this measurement device has some equivalence classes of macroscopic states which are distinguishable with near certainty. It also has states that don’t fall into one of those equivalence classes, but as long as it is in such an unclear state (like a superposition of a dead and an alive cat), it does not yet make sense to talk of a measurement.

This description has not given up (3) “the assumption that measurements have a single outcome”, because it only talks about measurement if there actually was a single macroscopic measurement result with near certainty.
It neither affirmed nor explicitly denied (2) “that the quantum state evolves linearly”. OK, it denied it for the quantum state of the smallest system, retreated to “approximately linear” for the middle system, and claimed to stay agnostic for the environment. So it basically gave up the assumption that the “relevant quantum states evolve exactly linearly”.
It claimed that the quantum state of the middle system (1) “encodes everything about the relevant physics”, where “relevant” has to be understood in an appropriate way.

Now you might claim that this analysis proves that Maudlin’s trilemma applies to this carricature, because point (2) did apply. But this misses the actual weak points of the carricature: (a) The “equivalence classes of macroscopic states which are distinguishable with near certainty” are subjective. The equivalence classes are not given objectively by themselves. (b) No indication is given why a measurement should actually occur, i.e. why the measurement device should evolve into one of the “macroscopic states which are distinguishable with near certainty”.

20. Mateus Araújo says:

Maudlin did formulate his trilemma very carefully. You could just read his paper, I linked it above.

I don’t see what is the point you’re trying to make. If you do go for option 2 you can solve the measurement problem, this is well known. Now if whatever you propose actually does it or suffers some other problem is besides the point.

21. gentzen says:

My point was that it was unclear to me how “carricature decoherence setups” based on three nested systems were supposed to be matched to Maudlin’s trilemma. There are three states (two of them definitely quantum states) in the carricature, hence it was unclear to me which of those three states Maudlin intended to talk about. And it was also unclear to me what would count as measurement for him. Note that in the carricature, a measurement has only happened after the measurement device evolved into one of the equivalence classes of macroscopic states which are distinguishable with near certainty.

After reading the article, I learned that what counts as measurement for Maudlin is the interaction itself between the smallest system (which is observed) and the measurement device. (This part I guessed wrong in my previous comment.) I also learned that the quantum state Maudlin intends to talk about is the quantum state of the middle system. (This part I guessed right in my previous comment.)

I agree that the article itself is clear and very carefully formulated. The article also contains two explicit versions of the trilemma, but only the third point about measurements is formulated significantly more careful than your formulation. (The main difference between the two explicit versions is in this third point, which is probably the reason why Maudlin tried to formulate it very carefully.) Here are Maudlin’s two explicit versions of his trilemma:

The following three claims are mutually inconsistent.
The wave-function of a system is complete, i.e. the wave-function specifies (directly or indirectly) all of the physical properties of a system.
The wave-function always evolves in accord with a linear dynamical equation (e.g. the Schrödinger equation).
Measurements of, e.g., the spin of an electron always (or at least usually) have determinate outcomes, i.e., at the end of the measurement the measuring device is either in a state which indicates spin up (and not down) or spin down (and not up).

Formally, the following three claims are mutually inconsistent:
The wave-function of a system is complete, i.e. the wave-function specifies (directly or indirectly) all of the physical properties of a system.
The wave-function always evolves in accord with a deterministic dynamical equation (e.g. the Schrödinger equation).
Measurement situations which are described by identical initial wave-functions sometimes have different outcomes, and the probability of each possible outcome is given (at least approximately) by Born’s rule.

Independent of whether those two explicit versions would be enough to identify the measurement part as the weak point of the carricature, reading the article itself makes it clear that this would be Maudlin’s conclusion (which would be spot on). His analysis and proofs however don’t apply directly to the carricature. Still they do provide a good starting point for an analysis which does apply to the carricature.

22. Jacques Pienaar says:

@Yash Sharma:

Your question about quantum computing hits close to home for me. In the end, the ultimate test of whether an interpretation is accepted by the physics community is not whether it is right or wrong in some abstract philosophical way, but whether it can naturally explain the things we find in our laboratories. I don’t think QBism is likely to be proven wrong, absurd, or inconsistent on purely logical grounds — it is too sophisticated for that. The real danger to QBism is actually of an empirical nature (a fact that our critics seem unable to grasp): if quantum theory breaks down at some scale, then QBism will be unable to explain that, and would be thrown out. And even if quantum theory holds at all scales, QBism might still fail, if it cannot provide intuitive explanations for important quantum phenomena. For without that, it would never attract the attention of most of the physics community and will eventually be forgotten.

The question of why quantum computers give an advantage over classical computers is exactly the kind of thing we would hope be answerable by appealing to the ontology of quantum theory. On this particular issue, many-worlds seems to have the upper hand so far: folklore has it that David Deutsch was inspired to invent quantum computing because the parallel universes suggested to him an analogy with parallel processing in computation. There are subtleties with the analogy (we can’t simply access those parallel worlds without restrictions), but many people do see this as a point in favour of many-worlds. Can QBism answer to that?

I don’t know yet, but think QBism can definitely give us a different perspective on computation in general. We are used to thinking of computation as something akin to a property of a physical system. But QBists would say, along with Turing, that computation is primarily a human activity. Turing’s famous machine was not supposed to be an abstraction of a physical system — it was supposed to be an abstraction of a particular kind of human activity. It was Deutsch who placed the emphasis on physical systems. Of course, he would not have made a distinction between ‘human activity’ and ‘a physical system obeying natural laws’ — that distinction would only make sense to a QBist. But that is precisely my point: for a QBist, the way that we think about computation would have to be revised to bring it more in line with Turing’s original conception. Then the way in which QBism would update the idea of a ‘Turing machine’ in light of quantum theory might take us down a different route than the one followed by Deutsch. I don’t know if that route would explain the quantum speedup, but I definitely think it would be interesting to pursue.