Is QBism simply a subjective interpretation of science?

[PreposterousUniverse]

Quantum Bayesianism takes the view that the there are no quantum states in the objective sense and that the probabilities should only be interpreted as what information an agent has about the system. Isn't this the same as claiming that there are hidden variables, and that probabilities arises only due to our ignorance about what properties the real physical system has? But at the same time they seem to claim that QBism is a local theory, which would be in a direct violation with Bell's theorem.

I don't understand how they rationalize this. How can they argue for hidden variables and at the same time claim their theory to be local?

 

[PeterDonis]

@PreposterousUniverse please give some specific references by QBIsm proponents that make the claims you describe.

 

[PreposterousUniverse]

"Second, it suggests that quantum mechanics can be thought of as a local theory, because the Einstein–Podolsky–Rosen (EPR) criterion of reality can be rejected."
From Wikipedia Quantum_Bayesianism

So supposedly, they claim QBism to be a local theory and at the same time claiming that the probabilities are only related to the agent's knowledge about the system. This to me seems to be a contradiction.

 

[PeterDonis]

From Wikipedia

Wikipedia is not a textbook or peer-reviewed paper. There are plenty of references to actual papers in the article. You should be reading those directly, not trusting Wikipedia to tell you what they say.

 

[PeterDonis]

Moderator's note: Thread moved to QM interpretations subforum.

 

[Demystifier]

QBism is local in a really weird way. First, QBists are sort of creationists. Second, if superluminal signaling was possible, QBism would still be a local interpretation. Let me explain it.

A QBist denies quantum nonlocality by the following argument. If one apparatus measures spin of one particle and another apparatus measures spin of another particle, we cannot speak of correlation until a single agent reads both results. Moreover, if the agent reads the spin of first particle at time t1 and the spin of second particle at time t2>t1, then for this agent the correlation does not exist before time t2. This is how QBism avoids nonlocality associated with the correlation. If fact, QBist denies that, for him, the result of measurement of the second spin was real before he seen it, at time t2.

But then, by the same token, when a QBist finds dinosaur bones, he should deny that, for him, the bones were real before he found them. Moreover, he should deny that the bones are evidence that the dinosaurs existed a long time ago. In this way, a QBist denies evidence for biological evolution. It means that QBists believe in a kind of creationism (in which the creator is each agent, not a universal God).

QBism is not really an interpretation of quantum mechanics, it is an interpretation of science. It proclaims that scientific knowledge is just a subjective knowledge of a (human-like) agent. The locality of QBism stems from the fact that any agent is local. Even if two agents could communicate superluminally or instantaneously, it wouldn't change the fact that each agent by itself is a local being.

The point of these arguments is to show that locality of QBism is a triviality not worthwhile of further scrutiny.

 

[vanhees71]

The entire QBism approach seems to me a rather philosophical (or even religious?) reinterpretation of the meaning of probabilities in general. We had once a Qbist in the colloquium, so that I could squeeze him about, what's the "meaning of a probability for a single outcome of a random experiment/quantum meaurement". He couldn't answer my question at all. Of course, if I say the probability to win next Saturday's lotery is some very small number, then after the fact, I've won or not won after the drawing, what does this single random experiment tell me about the validity of my estimate of the probability? I never understood, how the Baysenists and their quantum siblings get out of the dilemma that probabilities have a meaning in the empirical sciences as describing "expectations for the frequencies of the outcome of measurements", and that the probabilities can only be tested with some given significance level by preparing and measuring "sufficiently large ensembles".

 

[gentzen]

Ideas Abandoned en Route to QBism by Blake C. Stacey talks about an opinion piece by Peierls

The Fuchs–Peres essay has some affinities with the Rudolf Peierls opinion piece from a decade earlier [79] that they discussed during the writing process [63]. Mermin observes that Peierls would sound more QBist than perhaps any other figure from the early generations of quantum physicists, if he had not used the first-person plural collectively. Instead, he propagated the old confusions. This applies with equal force to the Fuchs–Peres collaboration.

[79] R. Peierls, “In defence of ‘measurement’,” Physics World 4 (January 1991), 19–20.

That opinion piece is very revealing, especially when it comes to making sense of the collapse of the wavefunction, and how to avoid the apparent non-locality it seems to imply (basically in the way sketched above by Demystifier). But it also has many passages that show that QBism is indeed only a "more honest" version of the interpretation put forward by defenders of the old Cohenhagen orthodoxy:

The objection is sometimes made: "How can one apply quantum mechanics to the early Universe, when there were no observers around?" The answer is that the observer does not have to be contemporaneous with the event. We can, from present evidence, draw conclusions about the early Universe, the classical example being the cosmic microwave background. In this sense we are observers. If there is a part of the Universe, or a period in its history, which is not capable of influencing present-day events directly or indirectly, then indeed there would be no sense in applying quantum mechanics to it.

 

[vanhees71]

Well, I must say, that it seems to me that Peierls is more clear than many modern writers of "opinion pieces" about these (apparent) "problems" of QT rather than "propagating old confusions". For me it's rather the qubists who add "new confusions" to the issues!

 

[martinbn]

QBism is local in a really weird way. First, QBists are sort of creationists. Second, if superluminal signaling was possible, QBism would still be a local interpretation. Let me explain it.

A QBist denies quantum nonlocality by the following argument. If one apparatus measures spin of one particle and another apparatus measures spin of another particle, we cannot speak of correlation until a single agent reads both results. Moreover, if the agent reads the spin of first particle at time t1 and the spin of second particle at time t2>t1, then for this agent the correlation does not exist before time t2. This is how QBism avoids nonlocality associated with the correlation. If fact, QBist denies that, for him, the result of measurement of the second spin was real before he seen it, at time t2.

But then, by the same token, when a QBist finds dinosaur bones, he should deny that, for him, the bones were real before he found them. Moreover, he should deny that the bones are evidence that the dinosaurs existed a long time ago. In this way, a QBist denies evidence for biological evolution. It means that QBists believe in a kind of creationism (in which the creator is each agent, not a universal God).

QBism is not really an interpretation of quantum mechanics, it is an interpretation of science. It proclaims that scientific knowledge is just a subjective knowledge of a (human-like) agent. The locality of QBism stems from the fact that any agent is local. Even if two agents could communicate superluminally or instantaneously, it wouldn't change the fact that each agent by itself is a local being.

The point of these arguments is to show that locality of QBism is a triviality not worthwhile of further scrutiny.

The bones in your example correspond to the particles, not the spin of the particles. Your example does not have an observable like the spin. So your example is actually meaningless.