The Ontology of Many‐Worlds : Modality and Time

Philippe Grangier

Alexia Auffèves, Philippe Grangier

Olímpia Lombardi

Valia Allori

Milena Radović

The purpose of this paper is to give an overview of modal realism and many worlds interpretation of quantum mechanics and compare the concepts of many worlds in those two theories. Main source for modal realism is David Lewis' book On the Plurality of Worlds (1986). For many worlds interpretation I rely on Bryce DeWitt and Lev Vaidman. First part of the paper concerns modal realism, second is about many worlds interpretation and third is the comparison of those two. Many worlds in modal realism and many worlds interpretation have the same phenomenology, but are different ontologically. The worlds of modal realism are causally and spatiotemporally separated and have always been, while the worlds of many worlds interpretation are part of a branching universe. That means they were the same world at some point in the past, even though they are equally separated, as the worlds of modal realism are, after the branching.

Claudio Calosi

The paper investigates the existence and the nature of quantum indeterminacy in a particular realist interpretation of quantum mechanics, that is, the Modal Hamiltonian Interpretation (MHI)-together with an ontology of properties for quantum systems. In doing so, it serves a twofold purpose. First, it advances the debate on quantum ontology by showing precisely how and why indeterminacy arises in a quantum world-as described by the MHI. Second, it offers a naturalistic example of genuine metaphysical indeterminacy, an example coming from our best physics.