Horizons are Watching You

We show, quite generally, that if a massive (or charged) body is put in a quantum spatial superposition, the mere presence of a Killing horizon in its vicinity will eventually decohere the superposition. 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. In particular, we fully analyze the case of uniformly accelerating superposition in flat spacetime. From the Rindler perspective, this superposition is decohered due to a flux of "soft gravitons/photons" through the Rindler horizon with negligible (Rindler) energy. We show that, from the inertial perspective, the decoherence is due to radiation of high-frequency gravitons/photons to null infinity. Furthermore, this decoherence is distinct from — and larger than — the decoherence resulting from the presence of Unruh radiation. 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.