Killing Horizons Decohere Quantum Superpositions

We recently showed that the mere presence of a black hole will eventually decohere any superposition in its exterior. We now show that, more generally, decoherence of stationary superpositions will occur in any spacetime with a Killing horizon. This occurs because, in effect, the long-range field of the body is registered on the Killing horizon which, we show, necessitates a flux of "soft horizon gravitons/photons" through the horizon. The Killing horizon thereby harvests "which path" information of quantum superpositions and will decohere any quantum superposition in a finite time. We particularly analyze the case of a uniformly accelerating superposition in flat spacetime which, from the Rindler perspective, is decohered due “soft” Rindler gravitons/photons which, from the inertial perspective are “hard” inertial gravitons/photons at null infinity. We also analyze decoherence due to the presence of the cosmological horizon in de Sitter spacetime. We provide estimates of the decoherence time for such quantum superpositions in both the Rindler and cosmological cases.

Coauthors: Robert M. Wald and Daine Danielson

arXiv:2301.00026 [hep-th]