The De-Relationalizing of Relational Quantum Mechanics

Abstract

A recent phase transition in the relational interpretation of quantum mechanics (RQM) is situated in its historical context, and the novelty of the post-transition viewpoint is questioned.Relational Quantum Mechanics (RQM) is an approach to interpreting quantum theory first proposed by Rovelli in the Before Times [1]. Recently, RQM has been critiqued by several authors [2][3][4][5][6][7]. Perhaps in response to some of these critiques, Adlam and Rovelli published a significant revision to RQM which, in so many words, backs away from the relationalism that had characterized the interpretation [8]. The change can be illustrated by a thought-experiment in the vein of Wigner's Friend, or the "observer observed". Bob stands outside a room, which he believes to contain Alice and a system, e.g., a qubit. We can contemplate two different kinds of consistency conditions regarding this scenario. Suppose that Bob can measure either Alice, the qubit, or both in succession. Bob might ask Alice, "What did you get when you measured the qubit in the σ z basis?" He might then expect that if he measures the qubit in the σ z basis himself, he will get the same answer that he heard from Alice. This is a first-person consistency condition: It is phrased entirely in terms of what Bob experiences and expects. We can also imagine a third-person kind of consistency, which is not about Bob's sense data, but rather about facts generally available. A third-person consistency condition might say that Alice's measurement outcome is a factfor-all, and Bob is guaranteed to get the same value if he performs the same measurement. Previously, Rovelli had endorsed a premise of Relativity of comparisons: it is meaningless to compare the accounts relative to any two systems except by invoking a third system relative to which the comparison is made.