Relational Quantum Mechanics is Still Incompatible with Quantum Mechanics

Abstract: We showed in a recent article (Lawrence et. al., 2023, Quantum 7, 1015), that relative facts (outcomes), a central concept in Relational Quantum Mechanics, are inconsistent with Quantum Mechanics. We proved this by constructing a Wigner-Friend type sequential measurement scenario on a Greenberger-Horne-Zeilinger (GHZ) state of three qubits, and making the following assumption: "if an interpretation of quantum theory introduces some conceptualization of outcomes of a measurement, then probabilities of these outcomes must follow the quantum predictions as given by the Born rule." Our work has been criticized by Cavalcanti, Di Biagio, and Rovelli (CDR). In this note we show that their critique is invalid, and that their specific arguments raise questions of principle.