Quantum Systems Other Than the Universe

TL;DR

This paper explores the interpretation of quantum theories beyond the universe, emphasizing that quantum systems are generally mixed and non-unitary, but unitary dynamics remain fundamental due to reductionist principles and empirical support.

Contribution

It develops a perspective on quantum systems as autonomous but not isolated, advocating for mixed states and non-unitary dynamics in general, while reaffirming the importance of unitary evolution in microscopic physics.

Findings

Quantum systems are best viewed as having mixed states and non-unitary dynamics.

Unitary dynamics remains fundamental due to reductionist principles and empirical evidence.

The position contrasts with the 'Open Systems View' by Cuffaro and Hartmann.

Abstract

How should we interpret physical theories, and especially quantum theory, if we drop the assumption that we should treat it as an exact description of the whole Universe? I expound and develop the claim that physics is about the study of autonomous, but not necessarily isolated, dynamical systems, and that when applied to quantum mechanics this entails that in general we should take quantum systems as having mixed states and non-unitary dynamics. I argue that nonetheless unitary dynamics continues to have a special place in physics, via the empirically-well-supported reductionist principles that non-unitarity is to be explained by restriction to a subsystem of a larger unitary system and that microscopic physics is governed by unitary and largely known dynamics. I contrast this position with the `Open Systems View' advocated recently by Michael Cuffaro and Stephan Hartmann.