Measurements and Elements of Reality in Relational Quantum Mechanics

Abstract

In Relational EPR, Rovelli and Smerlak argued that the EPR correlations do not entail any form of ‘nonlocality’ when interpreted within the framework of the relational interpretation of quantum mechanics by departing from a strict definition of realism, according to which physical systems exist ‘by themselves’ in the sense that they are characterised by an objective, absolute state of affairs in favour of an ontology of ‘sparse, relative facts’. I claim that Rovelli implicitly distinguishes between ‘direct’ and ‘indirect’ measurements: in the case of a “direct” measurement, the experimenter would not be able ascribe an element of reality to the particle but only to the reading of his apparatus, because the interaction between the particle and the measuring device would be independent from the interaction between the ‘observer’ and his device. I disagree with such a distinction and maintain that all measurements in quantum theory are always indirect measurements, in the sense that we always perform certain manipulations on a physical system to predict a value of another physical system that has previously interacted with the former. In this thesis, I analyse the implication of this claim with respect to Rovelli’s treatment of the EPR correlations and maintain that elements of reality can be retrospectively ascribed, without violating locality, to particles that have previously interacted with our measuring device by reconstructing a causal story that brought the measurement outcome about.