Source
Di Biagio and Rovelli (2021), LIT-1666, §§2.1–2.3, pp. 7–11 of the journal text; read in NOTE-1226. It builds on THEORY-1075.
What was actually shown
Why not stable facts (§2.1, p. 7). Stable facts are the textbook link to the world, and are ubiquitous for us because decoherence is. But “stability is relational … Therefore one does not avoid relationalism by restricting to stable facts”, and “stability is only approximate in general. At no point the interference terms perfectly vanish.” “How stable does a fact need to be before it is real? And with respect to what systems does it have to be stable, in order to be real?” The alternative is to base the ontology on all relative facts: “it is about real facts of the world and relative probabilities of their occurrence … based on facts established at interactions and are labelled by physical contexts” (p. 8).
Frauchiger and Renner (§2.2.1, p. 8). Their assumption (C), “If W, applying quantum theory, concludes that F knows that L_S = a, then W can conclude that L_S = a”, becomes: W may reason as if a fact relative to F were relative to W. That “holds only if every fact relative to F is stable for W”, and with full quantum control of the friend’s lab, “by definition, what is stable for the friends is not stable for W. Hence the contradiction follows from inappropriately mixing contexts”. That no contradiction follows when the friends’ labs decohere for W too is credited to [26, 27, 38].
Local Friendliness (§2.2.2, p. 9). Bong et al. show that absoluteness of observed events, no superdeterminism and Bell locality imply inequalities quantum theory violates, and the violation has been observed with photon friends. “Absoluteness of observed events means that if W deems that F is an observer, then W can use (3)—even if W has full quantum control on F”, which RQM denies. Dismissing photon friends “opens the problem of deciding which systems give rise to facts”.
The measurement problem and macroreality (§2.3, p. 10). Projection and unitary evolution “refer to facts relative to different systems”. Absolute macroreality “only emerges approximately, relative to systems for which decoherence is sufficiently strong.” “Decoherence needs an ontology. Relative facts provide such a general ontology that is well defined with or without decoherence.”
What this does not say
- That the no-go theorems favour RQM over its rivals. Calling the Local Friendliness violation “evidence that the facts quantum theory deals with are facts relative to systems” (p. 9) outruns the theorem, which shows that one of three assumptions fails; QBism and Everett also deny absoluteness. The same overreach is noted for Rovelli 2021 (THEORY-621).
- That relative facts cohere across observers in general. “Wigner will always agree with his Friend about the value of L_S once he too interacts with them” (p. 10) is argued from one entangled state’s correlation (THEORY-1080).
- How an agent should reason about an experiment on himself. “has not been done within RQM yet” (pp. 8–9).
- That an ontology of stable facts is impossible. The argument is that its two questions have no satisfactory answer, not a proof that none could.
- What makes something a system, or a context. Both are taken as given.