# Observer-independence in the presence of a horizon

## TL;DR

This paper explores the limitations of observer-independent facts in quantum mechanics within the context of horizons, revealing a loophole in existing no-go theorems that depends on the ability to prove inertial frames and detect Unruh radiation.

## Contribution

It identifies a loophole in Brukner's no-go theorem for observer-independent facts that arises when horizons are present, emphasizing the role of inertial frame verification and Unruh radiation.

## Key findings

- Loophole in the no-go theorem due to horizons
- Proving inertial frames requires detecting Unruh radiation
- Some facts remain unknowable, challenging observer-independence

## Abstract

In the famous thought experiment known as Wigner's friend, Wigner assigns an entangled state to the composite quantum system consisting of his friend and her observed system. In the context of this thought experiment, Brukner recently derived a no-go theorem for observer-independent facts, i.e. those facts that would be common to both Wigner and his friend, and initial experimental tests have given it strong support. In this article, we demonstrate a loophole in the theorem that manifests in the presence of a horizon even if Wigner and his friend are on the same side of the horizon. The loophole requires that Wigner and his friend both unambiguously demonstrate that they are in inertial frames. We then argue that unambiguously proving that they are both in inertial frames requires unambiguously proving the existence of Unruh radiation. However, we also argue that if they cannot prove this, then there are still limitations on observer-independent facts that require that some of those facts remain unknowable. As such, observer-independent facts remain illusory.