Fuchs ea QBism

Fuchs, Mermin, Schack An Introduction to QBism with an Application to the Locality of Quantum Mechanics arXiv.1311.5253

QBism – Quantum Bayesianism – is a relativistic variant of Quantum Mechanics. One has Bayesian agents who can report to each other observations and beliefs, with no ‘behind the scenes’ coherence constraints.

More conventionally, if one supposes that all observations arise from some common ‘objective reality’ then one gets various well-known ‘paradoxes’. As the abstract says:

We note that [QBism] removes the paradoxes, conundra, and pseudo-problems that have plagued quantum foundations for the past nine decades. As an example, we show in detail how it eliminates “quantum non locality”.

It achieves this at the expense of EPR’s reality criterion:

If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity.

It is claimed that:

QBism provides a powerful validation of the personalist view of probability.


This makes a valuable contribution to the continuing debate about the interpretation of QM and its spookiness. Removing objectivity (‘realism’) as an assumption (which even its originators did not regard as necessary) and replacing it with a relativistic formulation seems long overdue. The link between QBism and the personalist view of probability also seems useful. But although both ‘validate’ the other in a certain technical sense, this does not mean that either is true.

If a QBist agent starts with a belief in the form of a probability distribution, then this would be maintained ‘rationally’, in the usual way. But according to Bayes’ rule the change in belief is due to the likelihood function. Thus one could envisage a more tentative agent simply maintaining likelihoods and acting on those: the use of Bayesian priors does not seem essential. What seems to matter is the relativism of the formulation, not the precise form of the decision rule.

My understanding is that this distinction is important in the development of physics. For example, Kant argued ‘logically’ that our notion of space-time was necessarily Euclidean, so according to the EPR reality criterion (for Kant) ‘there exists an element of physical reality corresponding to’ Euclidean space-time. This is nonsense. A more straightforward interpretation of the criterion, in line with the original usage, is that whoever believes in a physical quantity can believe in the corresponding physical reality without introducing any additional inconsistencies. This is much less harmless and actually consistent with QBism.

If Kant were a QBist agent then he would never be enlightened, since his prior belief was that space-time was Euclidean, and no amount of evidence could over turn such a certainty. But if Kant took account of the likelihoods, he might admit that the evidence was increasingly against his ‘certain’ belief’ and entertain other hypotheses, in violation of Bayes’ rule. Similarly for other unanticipated phenomena, such as x-rays.

It would seem a good idea to separate the relativism from the Bayesianism.

See Also

My notes on uncertainty, including variations on Bayesianism.

Dave Marsay

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