I’ve long been interested in Carlo Rovelli’s Relational Quantum Mechanics (RQM), and had been aware that philosopher of science Bas C. van Fraassen had an unpublished paper discussing RQM. Recently, I saw that a preprint draft of this paper, called “Rovelli’s World,” had appeared on his website.
In the opening paragraph, van Fraassen calls RQM an “inspiring” original vision, and says “its presentation involves taking sides on a fundamental divide within philosophy itself.” Unfortunately, he doesn’t return explicitly to this last statement (and there is no conclusion section in the paper), but it is pretty clear that the key controversy of RQM revolves around the issue of realism. RQM seeks a consistent and complete interpretation of a quantum mechanical world, but this comes at the expense of fully objective realism. We give up the idea of absolute observer-independent quantum states, likewise observer-independent values of physical quantities; “the theory describes only the information systems have about each other.”
The main content of van Fraassen’s paper is a careful exercise in analyzing RQM to see what higher-order aspects of the world it describes are actually “absolute” (or objectively known) even as the states and measurement outcomes only exist relationally. He wants to compare what Rovelli - qua the author of the paper on RQM - seems to know about the world, as opposed to what a particular system in the world (playfully denoted “ROV”) can know, assuming the theory is correct. He looks at length at a specific example, where ROV is a third observer following on a “Wigner’s friend”-style example: based on his analysis he concludes an additional postulate should be added to RQM to clarify the scheme.