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

TL;DR

This paper unifies black hole physics and Wigner's friend scenarios to argue that no post-quantum theory can reconcile black hole information preservation with quantum predictions if no observer can falsify them.

Contribution

It constructs new paradoxes merging black hole physics with extended Wigner's friend scenarios, sharpening the cloning and firewall paradoxes to challenge post-quantum theories.

Findings

No post-quantum theory can reconcile black hole information preservation with quantum predictions.

Constructed paradoxes show the limits of alternative theories in black hole physics.

Highlights subtleties in assumptions used in black hole information puzzles.

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.