Density Matrix Realism

TL;DR

This paper advocates for Density Matrix Realism, proposing that the universal quantum state can be an impure density matrix, which has implications for scientific realism and differs from Wave Function Realism.

Contribution

It introduces and elaborates on Density Matrix Realism, contrasting it with Wave Function Realism, and explores its empirical equivalences and philosophical implications.

Findings

Density Matrix Realism allows for an impure universal quantum state.

Under certain conditions, Density Matrix Realism is empirically equivalent to Wave Function Realism.

Implications for scientific realism are discussed.

Abstract

Realism about quantum theory naturally leads to realism about the quantum state of the universe. It leaves open whether it is a pure state represented by a wave function, or an impure (mixed) one represented by a density matrix. I characterize and elaborate on Density Matrix Realism, the thesis that the universal quantum state is objective but can be impure. To clarify the thesis, I compare it with Wave Function Realism, explain the conditions under which they are empirically equivalent, consider two generalizations of Density Matrix Realism, and answer some frequently asked questions. I end by highlighting an implication for scientific realism.