I Making Sense of QBism: Non-Mathematical Reality & Reforming the Theory

[Demystifier]

TL;DR Summary

QBism as presented by QBists doesn't make sense (to me), so I reformulate QBism in a way that makes more sense.

I don't like QBism, for the reason it seems self-contradictory to me. There are at least 3 self-contradictions in the QBism literature:
(i) Sometimes it denies the existence of objective reality, but sometimes it accepts the existence of objective reality.
(ii) Even though it sometimes denies the existence of objective reality, it always denies solipsism.
(iii) Even though it sometimes accepts the existence of objective reality, it always denies its nonlocality proved by the Bell theorem.

Is there a way to reformulate QBIsm in a way that is not self-contradictory? I think there is, in a way first suggested to me by @DarMM. Here I want to elaborate this idea in my own terms.

In the spirit of Wigner's unreasonable effectiveness of mathematics in natural sciences, the main thesis is that objective reality exists, but cannot be described mathematically. Mathematics is nothing but a useful human construct. It may help us to think about the nature that surrounds us, but it is not the nature itself. Mathematical description of the laws of physics is a map, not the territory. The nature itself, the reality itself, is non-mathematical. Hence any mathematical theorem on reality (Bell, PBR, ...) is irrelevant and misleading, very much like a mathematical theorem about the existence or non-existence of God would be irrelevant and misleading. The only thing which can be described mathematically is our subjective knowledge, because mathematics is nothing but one of human ways of gaining subjective knowledge.

It's not that I am very happy with the interpretation above, but I cannot think of any better version of QBism that would make at least a little sense to me. If QBism makes sense at all, then it's only this version of QBism. Any thoughts?

 

[akvadrako]

I think the best way to interpret QBism is as a single-user theory. No matter what kind of external reality there is, an agent can use it to predict his experience. It isn’t solipsism but it isn’t not solipsism either; just silent on how other users experience the world.

Where they seem to go wrong is making claims about objective reality which don’t logically follow from the single-user context. Saying reality can’t be described mathematically is one example of this.

 

[Quanundrum]

In my humble opinion QBism is simply a fancy way of mystifying "Shut up and calculate"

 

[Demystifier]

In my humble opinion QBism is simply a fancy way of mystifying "Shut up and calculate"

Or perhaps QBism is way to replace "consciousness" (as in the von Neumann idea that collapse is induced by consciousness) with something that is equally vague but sounds less mysterious, that is - "information".

 

[Quanundrum]

Or perhaps QBism is way to replace "consciousness" (as in the von Neumann idea that collapse is induced by consciousness) with something that is equally vague but sounds less mysterious, that is - "information".

Indeed, whenever I've attempted to pin point anything concrete in QBism it has been futile, it just evolves into a game of semantics. It reminds me a lot of Wheeler's concepts of "It from Bit". It sounds all profound and interesting, but it's so nebulous that any attempt to get 2 different people to agree on anything concrete seems impossible.

 

[Minnesota Joe]

In the spirit of Wigner's unreasonable effectiveness of mathematics in natural sciences, the main thesis is that objective reality exists, but cannot be described mathematically. Mathematics is nothing but a useful human construct. It may help us to think about the nature that surrounds us, but it is not the nature itself. Mathematical description of the laws of physics is a map, not the territory. The nature itself, the reality itself, is non-mathematical. Hence any mathematical theorem on reality (Bell, PBR, ...) is irrelevant and misleading, very much like a mathematical theorem about the existence or non-existence of God would be irrelevant and misleading. The only thing which can be described mathematically is our subjective knowledge, because mathematics is nothing but one of human ways of gaining subjective knowledge.

Can you elaborate on how this helps QBists who are objective realists? On that view subjective knowledge is a subset of objective reality. How can they avoid theorems on objective reality themselves?

 

[Demystifier]

On that view subjective knowledge is a subset of objective reality.

If you are a mind-body dualist https://en.wikipedia.org/wiki/Mind–body_dualism,
then subjective knowledge (the mind) is not a subset of objective reality (the body).

 

[Minnesota Joe]

If you are a mind-body dualist https://en.wikipedia.org/wiki/Mind–body_dualism,
then subjective knowledge (the mind) is not a subset of objective reality (the body).

But they interact, an infamous problem for dualism. And it seems more acute for your QBist since this isn't solipsism and since objective reality somehow impinges on the mental. So, I don't see how they can coherently maintain that mathematics can't map objective reality. Maps needn't be perfect. They still might contain useful sign and symbols. Like "Here be non-locality". And dismissing the territory as unmappable is no argument against the non-local anyway.

 

[eloheim]

I have to agree with Demystifier and others here that QBism has always been a tad baffling, tbh. It's hard for me to understand its core identity beyond "we don't like any of the other qm interpretations out there." I'm also not sold on using math/science to prove that something can't be described mathematically/scientifically.

I'd peg the contention that "qm can only work as a single-user theory" as QBisms most interesting kernel, but I'm also deeply suspicious of any theory that tries to give special significance to the conscious(?) or intelligent agent, not least of all because any human "individual" can just as accurately be described by an infinity of different partitions and combinations, and, in turn, there's no known physical reason for separating an individual from the greater environment in the first place.

In other words, if I (THE agent) can model another human being, including their physiology and interaction with the environment, with perfect fidelity, then what part of that agent's experience could remain insufficiently described? It's hard for me to think of a place to go with that, outside of appealing to extra-scientific concepts such as mind, soul, etc. To my mind, a person (composed of quanta) should be described by quantum physics like anything else. And if the problem is that a quantum universe can only be consistently modeled from a single perspective, then why not just (e.g.) use the POV of a (real or imagined) third-party, to encompass both "you" and "I" in the same set of equations?

I read quite a few papers on QBism (both pro and con) when the interpretation first came to my attention, but I never really felt like I fully understood what it was, much less how it's supposed to work.

 

[martinbn]

Summary: QBism as presented by QBists doesn't make sense (to me), so I reformulate QBism in a way that makes more sense.

I don't like QBism, for the reason it seems self-contradictory to me. There are at least 3 self-contradictions in the QBism literature:
(i) Sometimes it denies the existence of objective reality, but sometimes it accepts the existence of objective reality.
(ii) Even though it sometimes denies the existence of objective reality, it always denies solipsism.
(iii) Even though it sometimes accepts the existence of objective reality, it always denies its nonlocality proved by the Bell theorem.

Can you elaborate on these. Where does QBism deny the existence of objective reallity? Or where does it deny Bell nonlocality?

 

[Demystifier]

But they interact, an infamous problem for dualism.

If they interact, it still doesn't mean that one is subset of the other.

 

[Demystifier]

Where does QBism deny the existence of objective reallity?

https://www.quantamagazine.org/quantum-bayesianism-explained-by-its-founder-20150604/"My fellow QBists and I instead think that what Bell’s theorem really indicates is that the outcomes of measurements are experiences, not revelations of something that’s already there. Of course others think that we gave up on science as a discipline, because we talk about subjective degrees of belief. But we think it solves all of the foundational conundrums."

Or where does it deny Bell nonlocality?

http://de.arxiv.org/abs/1810.13401Item 2.

 

[martinbn]

https://www.quantamagazine.org/quantum-bayesianism-explained-by-its-founder-20150604/"My fellow QBists and I instead think that what Bell’s theorem really indicates is that the outcomes of measurements are experiences, not revelations of something that’s already there. Of course others think that we gave up on science as a discipline, because we talk about subjective degrees of belief. But we think it solves all of the foundational conundrums."http://de.arxiv.org/abs/1810.13401Item 2.

I have to disagree with this. The first doesn't deny the existence of objective reality. It denies the the prexisiting values. Just as it is in standard QM ala Bohr. The second doesn't deny Bell nonlocality, it denies nonlocality (in the ususal sense of the word) by saying that QBism is local.

Does this resolve the paradoxes you had in mind?

 

[akvadrako]

I have to disagree with this. The first doesn't deny the existence of objective reality. It denies the the prexisiting values. Just as it is in standard QM ala Bohr.

In QBism the quantum state is subjective and also everything the theory encompases. There is no property or description which can be applied to objective reality.

The second doesn't deny Bell nonlocality, it denies nonlocality (in the ususal sense of the word) by saying that QBism is local.

Right – it doesn't deny Bell nonlocality, but it denies that locality is the mistaken assumption in Bell's theorem. Because when Alice measures up, she knows that Bob will measure up. But that isn't an objective fact about the world which Bob can use, it's just information localized in her head.

There are no objective facts of any kind described by QBism; objective in the sense of shared by multiple agents.

 

[Demystifier]

Does this resolve the paradoxes you had in mind?

No.

 

[martinbn]

No.

Why not? It seems that they stem from the claim that QBism denies the existence of objective reality, which may be the case, but the cited passages don't show that. What you quoted says that there are no preexisting values of unmeasured observables. That is not rejecting objective reality. @akvadrako said that in QBism there is no objective description of reality, but again that is not to say that there is no objective reality. So, I still don't see why you think that there are paradoxes?

 

[Demystifier]

Why not? It seems that they stem from the claim that QBism denies the existence of objective reality, which may be the case, but the cited passages don't show that.

The quote says that measurement outcomes are experiences. It does not define "experience", but in my dictionary experience is something subjective, not objective.

But even more explicit is the paper https://arxiv.org/pdf/1311.5253.pdf . Here are some quotes that demonstrate it (the boldings are mine):
"A QBist takes quantum mechanics to be a personal mode of thought — a very powerful tool that any agent can use to organize her own experience."
"QBist quantum mechanics is local because its entire purpose is to enable any single agent to organize her own degrees of belief about the contents of her own personal experience."
"Quantum correlations, by their very nature, refer only to time-like separated events: the acquisition of experiences by any single agent. Quantum mechanics, in the QBist interpretation, cannot assign correlations, spooky or otherwise, to space-like separated events, since they cannot be experienced by any single agent."
"QBism personalizes the famous dictum of Asher Peres. The outcome of an experiment is the experience it elicits in an agent. If an agent experiences no outcome, then for that agent there is no outcome. Experiments are not floating in the void, independent of human agency."
"And an outcome does not become an outcome until it is experienced by the agent. That experience is the outcome."
"Why, then, do many people wrongly claim that quantum mechanics is nonlocal? They do so by denying at least one of three fundamental precepts of QBism: ...
(3) Parameters that do not appear in the quantum theory and correspond to nothing in the experience of any potential agent can play no role in the interpretation of quantum mechanics."

 

[martinbn]

The quote says that measurement outcomes are experiences. It does not define "experience", but in my dictionary experience is something subjective, not objective.

Yes, but that is for the measurement outcomes, not for the reality behind it. This is no different to classical physics. Different observers will measure different velocity of an object (depending on how they move with respect to it), and will have a subjective outcome, but classical physics doesn't deny objective reality.

But even more explicit is the paper https://arxiv.org/pdf/1311.5253.pdf . Here are some quotes that demonstrate it (the boldings are mine):
"A QBist takes quantum mechanics to be a personal mode of thought — a very powerful tool that any agent can use to organize her own experience."
"QBist quantum mechanics is local because its entire purpose is to enable any single agent to organize her own degrees of belief about the contents of her own personal experience."
"Quantum correlations, by their very nature, refer only to time-like separated events: the acquisition of experiences by any single agent. Quantum mechanics, in the QBist interpretation, cannot assign correlations, spooky or otherwise, to space-like separated events, since they cannot be experienced by any single agent."
"QBism personalizes the famous dictum of Asher Peres. The outcome of an experiment is the experience it elicits in an agent. If an agent experiences no out come, then for that agent there is no outcome. Experiments are not floating in the void, independent of human agency."
"And an outcome does not become an outcome until it is experienced by the agent. That experience is the outcome."

Same for these.

 

[Demystifier]

Yes, but that is for the measurement outcomes, not for the reality behind it.

Yes, but QBism refuses to analyse this reality behind it. Moreover, it claims that it is wrong to analyse it, and hence that the outcome of such an analysis (that is, Bell nonlocality) is wrong. Here is a quote that supports it:
"Why, then, do many people wrongly claim that quantum mechanics is nonlocal? They do so by denying at least one of three fundamental precepts of QBism: ...
(3) Parameters that do not appear in the quantum theory and correspond to nothing in the experience of any potential agent can play no role in the interpretation of quantum mechanics."

 

[martinbn]

Yes, but QBism refuses to analyse this reality behind it. Moreover, it claims that it is wrong to analyse it, and hence that the outcome of such an analysis (that is, Bell nonlocality) is wrong. Here is a quote that supports it:
"Why, then, do many people wrongly claim that quantum mechanics is nonlocal? They do so by denying at least one of three fundamental precepts of QBism: ...
(3) Parameters that do not appear in the quantum theory and correspond to nothing in the experience of any potential agent can play no role in the interpretation of quantum mechanics."

I am still confused. That doesn't say that there is no reality. It only says that the analysis is subjective.

 

[Demystifier]

I am still confused. That doesn't say that there is no reality. It only says that the analysis is subjective.

Does my personal thinking about the Bell theorem, and my personal conclusion that objective reality must obey a nonlocal law, counts as subjective analysis?

 

[martinbn]

Does my personal thinking about the Bell theorem, and my personal conclusion that objective reality must obey a nonlocal law, counts as subjective analysis?

No, but your conlusion is wrong. The conclusion should be that obejective reality must obey Bell nonlocal laws. It doesn't have to be nonlocal in the usual sense of the word.

 

[Demystifier]

The conclusion should be that obejective reality must obey Bell nonlocal laws. It doesn't have to be nonlocal in the usual sense of the word.

What do you mean by the "usual sense"?

 

[martinbn]

What do you mean by the "usual sense"?

Finite speed of propagation.

 

[akvadrako]

@akvadrako said that in QBism there is no objective description of reality, but again that is not to say that there is no objective reality. So, I still don't see why you think that there are paradoxes?

QBists claim there is an objective reality, but that's just a unrelated personal belief; it doesn't follow from any of the other assumptions and it has no empirical consequences; one can't ascribe any measurable properties to it.

So their objective reality is compatible with being nothing! It's hard to see the difference between objective reality being equivalent to nothing and no objective reality. QBists can make further arbitrary claims about it but they might as well be describing Plato's mathematical realm; if those claims can't be proven there is no basis for them.

 

[Minnesota Joe]

QBists claim there is an objective reality, but that's just a unrelated personal belief; it doesn't follow from any of the other assumptions and it has no empirical consequences; one can't ascribe any measurable properties to it.

So their objective reality is compatible with being nothing! It's hard to see the difference between objective reality being equivalent to nothing and no objective reality. QBists can make further arbitrary claims about it but they might as well be describing Plato's mathematical realm; if those claims can't be proven there is no basis for them.

Can the QBist even consistently claim "There is an objective reality" or "There are other minds"? Is ascribing existence consistent?

 

[kith]

(i) & (ii): Sure, QBism is in principle compatible with the non-existence of an objective external world but the main thing QBism is about is the experience of the user. Just like I conclude from my experiences with physicsforums that the people who post in this thread are probably not in my head, most QBists conclude from their experiences in the lab that the things in the lab are probably not in their head.

(iii): I think it is mostly a matter of semantics whether you want to call it nonlocal or not. For me, the important point is what is undesireable about nonlocality? If I try to answer this question without using the word "nonlocal" itself, I would say something like "physical things showing FTL behaviour". This of course is absent in QBism (and in other Copenhagenish interpretations). So what would a succinct description of the problem with nonlocality in this case be?

 

[Demystifier]

most QBists conclude from their experiences in the lab that the things in the lab are probably not in their head.

By the same token, they should also conclude (through the Bell theorem) that things in the lab probably obey some nonlocal laws. And yet they refuse to conclude this.

 

[Demystifier]

So what would a succinct description of the problem with nonlocality in this case be?

Perhaps something like this. Objective reality exists, but quantum mechanics, according to QBism, is not a theory of objective reality. Likewise, objective reality obeys some nonlocal laws, but quantum mechanics is not a theory of those nonlocal laws.

 

[Demystifier]

Finite speed of propagation.

This is the definition of local. What is then the definition of nonlocal? Infinite speed of propagation? Communication without propagation at all? (Bohmian mechanics is nonlocal in the second sense.)

 

[kith]

Perhaps something like this. Objective reality exists, but quantum mechanics, according to QBism, is not a theory of objective reality. Likewise, objective reality obeys some nonlocal laws, but quantum mechanics is not a theory of those nonlocal laws.

That's a solution. I'm asking for a succinct description of the problem of nonlocality in the context of QBism and since the term "nonlocality" is so prone to misunderstandings I suggested not to use it in this description.

For realistic interpretations such a description is easy (see my last post) but I struggle to find one if the wavefunction describes experiences. For example what would something like a FTL speed of propagation of an experience even mean?

 

[akvadrako]

Can the QBist even consistently claim "There is an objective reality" or "There are other minds"? Is ascribing existence consistent?

This is why they claim those objects cannot be analyzed or described mathematically. Because of Bell's theorem, you need nonlocality, multiple outcomes, retro-causation, etcetera; they also exclude all those possibilities.

Does claiming a problematic result of a theory is off-limits to analysis make it consistent?

 

[atyy]

For example what would something like a FTL speed of propagation of an experience even mean?

Depends on the size of your head or whatever is experiencing the experience :p

 

[Stephen Tashi]

Is there a way to reformulate QBIsm in a way that is not self-contradictory?

What about getting rid of the notion that the Bayesian updating of probabilities (and the things used to calculate them) is "subjective"?

If a person has information $A$, he can "objectively" test how often a result $C$ happens by doing experiments where the situation described by $A$ is set up and the outcome is observed. If the person has information $A$ and information $B$, the person can objectively test how often a result $C$ happens by setting up an experiment where both $A$ and $B$ describe the situation. I see no reason to say that theory that gets different answers for $Pr(C|A)$ and $Pr(C | A \cap B)$ is "subjective".

In stories that descibe the actions of Bobs and Alices, we are told that once upon a time Bob knew this and Alice knew that, and perhaps at a later time, they knew something different. The idea that Bob and Alice behave "subjectively" if they know different things or calculate different probabilities as the story progresses seems to require that there is an "objective" reader of the story who has all the information and thus knows the "objectively" correct answer.

By analogy, if the reader of the story happened to have a ladder that was leaning against a wall, he could say that when Bob and Alice work problems in trigonometry texts about ladders leaning against walls, that they are being "subjective" if the calculated length of a ladder based on information given in different problems doesn't agree with the length of the reader's ladder.

 

[Demystifier]

I'm asking for a succinct description of the problem of nonlocality in the context of QBism

Maybe this. QBism says that unobservable quantities that are not a part of the mathematical formulation of quantum theory are irrelevant. For instance, objective reality is not a part of the mathematical formulation of quantum theory, so it's irrelevant. However, there is no such thing as the mathematical formulation of quantum theory. Instead, there are many formulations, such as Heisenberg picture, Schrodinger picture, path integrals, etc. But pilot waves (aka Bohmian mechanics) can also be viewed as one such mathematical formulation
https://pdfs.semanticscholar.org/2fb0/4475228ff385a44a16e3ba42b432d3bf5b17.pdf
so it is not clear why it would be irrelevant from the QBist point of view. But this formulation is explicitly nonlocal.

 

[kith]

But pilot waves (aka Bohmian mechanics) can also be viewed as one such mathematical formulation https://pdfs.semanticscholar.org/2fb0/4475228ff385a44a16e3ba42b432d3bf5b17.pdf so it is not clear why it would be irrelevant from the QBist point of view. But this formulation is explicitly nonlocal.

Thanks for this stimulating paper. I agree that all these mathematical formalisms somehow belong to the theory of QM itself. To me, this raises the interesting question how a mathematical formalism which is usually associated with a certain interpretation is interpreted in other interpretations. So how would a QBist talk about the equations of dBB or the path integral? I think you still dodged my question a bit by writing "But this formulation is explicitl nonlocal" (especially considering that I asked to taboo the word nonlocal ;-)).

 

[Minnesota Joe]

This is why they claim those objects cannot be analyzed or described mathematically. Because of Bell's theorem, you need nonlocality, multiple outcomes, retro-causation, etcetera; they also exclude all those possibilities.

Does claiming a problematic result of a theory is off-limits to analysis make it consistent?

You are saying that QBism evades inconsistency by claiming that objective reality can't be analyzed or described mathematically. If that is a fair characterization, I agree that is a reason for the evasion, but I wasn't asking about that type of inconsistency (tension with Bell's theorem).

Instead I was asking about about an inconsistency between the principle used to evade Bell and with the other beliefs they adopt, if they do, about the existence of objective reality and other minds.

 

[Demystifier]

So how would a QBist talk about the equations of dBB or the path integral? I think you still dodged my question a bit by writing "But this formulation is explicitl nonlocal" (especially considering that I asked to taboo the word nonlocal ;-)).

If I were a QBist who wants to avoid nonlocality at any cost, I would do it this way. Consider two particles in 3 spatial dimensions described by coordinates ${\bf x}_1,{\bf x}_2$. Mathematically it is the same as one particle in 6 spatial dimensions. From this 6 dimensional perspective, all the Bohmian equations are local in the 6-dimensional space. The 6-dimensional space, of course, is not what we experience in experiments, but that's not a problem for QBism. After all, the wave function $\psi$ is also not something that we experience in experiments, and yet nobody complains that it is a problem for QBism. Those mathematical objects are just abstract mathematical tools, they are not reality. In this sense QM is local even in the Bohmian formulation. A similar 6-dimensional perspective can also be taken for two particles in the path-integral formulation. And the generalization to $n$ particles is, of course, obvious.

 

[kith]

This doesn't seem very qbist in spirit to me. I think that they would rather answer along lines which are similar to section 21 in the 2019 FAQBism paper by DeBrota & Stacey which you cited earlier. There, they outline how they interpret unitary time evolution in terms of beliefs.

 

[Demystifier]

This doesn't seem very qbist in spirit to me. I think that they would rather answer along lines which are similar to section 21 in the 2019 FAQBism paper by DeBrota & Stacey which you cited earlier. There, they outline how they interpret unitary time evolution in terms of beliefs.

But in this text they do not deal with the Bohmian formulation of QM. The issue is how would a QBist interpret the Bohmian formulation of QM.

 

[kith]

But in this text they do not deal with the Bohmian formulation of QM. The issue is how would a QBist interpret the Bohmian formulation of QM.

If we treat the Bohmian equations (but not the Bohmian ontology) as part of the theory of QM and look for a QBist interpretation, I expect terms like "experience", "belief", etc. to play a prominent role. For the first part of the Bohmian equations (the Schrödinger equation), DeBrota & Stacey have given such an interpretation in section 21 of their paper, so it seems natural to me, to try to apply a similar scheme to the second part of the equations. I don't know if this can be done and maybe it can't but then I want to understand why. In any case, your account in post #38 doesn't try to go this route.

 

[Demystifier]

For the first part of the Bohmian equations (the Schrödinger equation), DeBrota & Stacey have given such an interpretation in section 21 of their paper, so it seems natural to me, to try to apply a similar scheme to the second part of the equations. I don't know if this can be done and maybe it can't but then I want to understand why. In any case, your account in post #38 doesn't try to go this route.

Perhaps a QBist may think of Bohmian trajectories as something analogous to gauge potentials. They may be useful as an additional computational tool, but they are not "physical" in the QBist sense (e.g. not directly encode beliefs on experiences). The nonlocality associated with Bohmian trajectories is analogous to nonlocality associated with the Coulomb gauge. Moreover, Bohmian velocities obey a kind of "gauge" symmetry, in the sense that the velocities ${\bf v}_{\rm Bohm}$ and ${\bf v}'_{\rm Bohm}$ make the same measurable predictions if they are related as
$${\bf v}'_{\rm Bohm}={\bf v}_{\rm Bohm}+{\bf u}$$
where ${\bf u}$ is an arbitrary solution of
$${\bf \nabla}(|\psi|^2{\bf u})=0$$

 

[kith]

Perhaps a QBist may think of Bohmian trajectories as something analogous to gauge potentials. They may be useful as an additional computational tool, but they are not "physical" in the QBist sense (e.g. not directly encode beliefs on experiences). The nonlocality associated with Bohmian trajectories is analogous to nonlocality associated with the Coulomb gauge.

Yes, that sounds better.

Now even though in electrodynamics, we often use the Coulomb gauge, nobody considers electrodynamics to be nonlocal (or at least does not consider nonlocality in unphysical computational tools a problem).

So if the QBist takes an analogous point of view about the Bohmian formalism of QM, why would she consider QM to be nonlocal (or consider nonlocality in these unphysical computational tools a problem)?

[Also thanks for pointing out the "gauge freedom" in the Bohmian velocities. I wasn't aware of this interesting fact.]

 

[PeterDonis]

why would one consider QM to be nonlocal

Because QM gives experimental results that violate the Bell inequalities. Classical EM does not.

 

[kith]

Because QM gives experimental results that violate the Bell inequalities. Classical EM does not.

The context of what you quoted was the QBist perspective. I edited my post to make this more clear.

I think we shouldn't conflate the specific discussion I have with @Demystifier (about how the Bohmian formulation of QM would be interpreted from a QBist perspective) with the broader question of why QBism is considered to be local by its proponents.

 

[Demystifier]

So if the QBist takes an analogous point of view about the Bohmian formalism of QM, why would she consider QM to be nonlocal (or consider nonlocality in these unphysical computational tools a problem)?

Even though quantum nonlocality cannot be used to send superluminal signals, in http://de.arxiv.org/abs/1006.0338 I argued that it can be used to create an illusion of sending superluminal signals. This illusion is experienced by an agent who does not know that her illusion of free will has been manipulated by an external manipulator. Perhaps a QBist might interpret it as a genuine nonlocality.

 

[kith]

Just a quick comment: I skimmed your paper and didn't understand its main point from this. At the moment, it's unlikely that I give it a more thorough reading. I think that in principle, questions of free will are related to quantum interpretations but I'm more interested in more tangible things.

 

[akvadrako]

In regards to how a subjective theory like QBism could be non-local:

Nominally, a subjective theory is always local because all the information is "in the head" of the observer: it includes everything he needs to know to make the best prediction possible. Let's assume his head is of a finite size and there is a bound on how much information it can contain. Then, if the information needed for a model of his experience with the maximally possible precision exceeds this bound, whatever is generating his experience must be storing and retrieving information non-locally.

Would that make sense to a QBist?

 

[Demystifier]

Then, if the information needed for a model of his experience with the maximally possible precision exceeds this bound, whatever is generating his experience must be storing and retrieving information non-locally.

Interesting! Do you have any example in which the bound could be exceeded?

 

[akvadrako]

Interesting! Do you have any example in which the bound could be exceeded?

Nothing specific. The best I can come up with is imagine an experimenter studying some system. He records lots of data, much more than he can remember even if he's a perfect information storage device. If more of those records are needed to compute the future observations of that system then can be encoded locally, what's physically local must be only an approximation of them. Some non-local information is physically relevant.

I don't know if this really makes sense.