How to Time Reverse a Quantum System

The received view of the meaning of time reversal in quantum mechanics suffers from a problem of conventionality. I review existing attempts by philosophers to avoid this problem, and argue that they fall short. In their stead, I propose an alternative approach to the meaning of time reversal in quantum theory inspired by Wigner. In particular, I show that a refinement of Wigner’s assumptions gives rise to several precise theorems beyond what Wigner himself realized, which completely characterize the meaning of time reversal, while avoiding the shortcomings of the received view.