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

[Demystifier]

TL;DR

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.)