Wigner's friend's black hole adventure: an argument for complementarity?

Abstract

At the heart of both black hole physics and Wigner's friend scenarios lies the question of unitarity. In Wigner's friend setups, sealed-lab measurements are modeled unitarily, probing the measurement problem. In black hole physics, the unitarity problem concerns information preservation in evaporation. We extend a recent analogy between these two puzzles exposed by Hausmann and Renner [arXiv:2504.03835v1] by constructing new paradoxes that merge black hole physics with extensions of the Wigner's friend scenario into a unified argument. This unified construction allows us to sharpen the cloning and firewall paradoxes, which leave room for a post-quantum theory to consistently describe the physics of black holes. We close this loophole, showing that no such theory exists if no observer can experimentally falsify quantum theory's predictions. We conclude by briefly highlighting subtleties in assumptions commonly used in black hole puzzles.