A new essay by David Mermin about QBism, an approach to quantum mechanics that focuses on individual observers and their probability distributions for observations. So let me raise the usual objection: what is the real world in this view? (1/n)
arxiv.org/abs/1809.01639
arxiv.org/abs/1809.01639
Epistemic approaches to QM posit that a wave function is a way of encapsulating our degrees of belief about what will happen. Fine, but it's still important to decide whether there is an objective external reality where things happen, or just agents talking to each other. (2/n)
Some quantum epistemicists deny the existence of objective reality straightforwardly. Others insist that it's there, it's just not what they are talking about, or need to talk about. But many of them adopt a frustratingly vague position. (3/n)
Mermin is in the vague camp. This is his clearest statement in the paper: of course there is an external world! But it's just something we all made up. (4/n) 
Other approaches to QM (Everett, dynamical collapse, hidden variables) all have explicit models of what objective reality is. So the move of not really talking about it is by no means forced on us by quantum mechanics. (5/n)
I'm not alone in believing that we have really good reasons to believe in objective reality. (Most obviously, our subjective experiences are way too compatible with each other otherwise.) (6/n)
But mostly it's frustrating that QBists seem to not want to address the question. (Many other epistemicists face it head-on, which is great.) I suspect if they did, they would find that workable models of reality look like other existing formulations of QM. (7/7)
(Of course it's possible that QBists have a clear and well-formulated idea of what reality is, they've just succeeded in hiding it from me for a long time.)
