Wikipedia has claimed for over two years now that John von Neumann was the "first quantum Bayesian." In context, this reads as stating that von Neumann inaugurated QBism, the approach to quantum theory promoted by Fuchs, Mermin and Schack. This essay explores how such a claim is, historically speaking, unsupported.This paper takes Wikipedia way too seriously.
Quantum Bayesianism, or QBism, is a modern defense of the Copenhagen interpretation, which was the mainstay of the XX century, but is often attack by popularist writers. Mermin advocates QBism partially by arguing that it is a modern new interpretation to compete with other modern interpretations, and partially by arguing that it is the same as what Bohr, Heisenberg, and Schroedinger promoted all along. In spite of his objections, articles on interpretations of quantum mechanics treat Copenhagen and QBism as essentially the same thing.
Von Neumann–Wigner interpretation is also a variant of Coperhagen defined by:
also described as "consciousness causes collapse [of the wave function]", is an interpretation of quantum mechanics in which consciousness is postulated to be necessary for the completion of the process of quantum measurement.This is a useful term, because a lot of people hate Copenhagen because of the role of consciousness. In this Wikipedia terminology, the von Neumann interpretation depends on consciousness, and the Copenhagen interpretation is the same thing with the consciousness stripped out.
Stacey refuses to say von Neumann was a QBist because he was not a Bayesian. There are multiple probability interpretations, and von Neumann was more of a frequentist.
At this point, you may wonder what the relation to physics is. Quantum mechanics successfully predicts atomic phenomena. What difference does it make how it is interpreted? The Feynman "shut up and calculate" school of thought says that interpretations are unnecessary distractions. When interpretations posit proliferating parallel unobservable universes, it is hard to see why there is any point to talking about such nonsense. If two interpretations have the same physical outcomes, it is hard to see how science can say one is better than the other.
Feynman did not actually say "shut up and calculate", but he did advocate having multiple theories for getting the same results, and criticize philosophical arguments about interpretations.
The metaphysical problem is weirder in the above paper, as the von Neumann and QBism interpretations are essentially the same physically and mathematically, but differ only in the interpretation of what a mathematical probability means. And that difference hinges on consciousness, whatever that is. Supposedly Wigner once remarked that a dog was probably sufficiently conscious to cause the collapse of a wave function (and kill Schroedinger's cat), but a rat was not. He must have realized that he was on thin ice with that.
I think that these interpretations add some clarity to quantum mechanics, but it is a mistake to take them too seriously. Some people think that quantum mechanics is a flawed theory, and it needs a new interpretation to save it. Or we need to get everyone on board some version of many-worlds. I disagree. The interesting questions are the scientific ones, and these interpretational issues barely qualify. We cannot use them in experiments, such as asking a dog to watch Schroedinger's cat and checking to see if the wave function collapses. An interpretation is just a way of thinking about the theory.