Black Holes, Entanglement and Decoherence

It was recently shown that a black hole (or any Killing horizon) will decohere any quantum superposition in their vicinity. I review three distinct but equivalent arguments that illustrate how this phenomenon arises: (1) entanglement with “degrees of freedom” in the interior (2) the absorption of soft, entangling radiation emitted by the superposition and (3) interactions with the quantum, fluctuating multipole moments of a black hole arising from ultra low frequency Hawking quanta. The relationship between “soft hair” and interactions with “internal degrees of freedom” is emphasized and some implications for the nature of horizons in a quantum theory of gravity are discussed.