Macroscopic superpositions and black hole unitarity

Abstract

We discuss the black hole information problem, including the recent claim that unitarity requires a horizon firewall, emphasizing the role of decoherence and macroscopic superpositions. We consider the formation and evaporation of a large black hole as a quantum amplitude, and note that during intermediate stages (e.g., after the Page time), the amplitude is a superposition of macroscopically distinct (and decohered) spacetimes, with the black hole itself in different positions on different branches. Small but semiclassical observers (who are themselves part of the quantum amplitude) that fall into the hole on one branch will miss it entirely on other branches and instead reach future infinity. This observation can reconcile the subjective experience of an infalling observer with unitarity. We also discuss implications for the nice slice formulation of the information problem, and to complementarity.

Figures (1)

• Figure 1: Schrodinger evolution of initial state $\Psi_i$, which describes a black hole progenitor and Alice. At intermediate times ($t > t_{\rm Page}$) $\Psi$ has ( unavoidably -- due to radiation recoil) decohered into two branches with the black hole in different locations. On one of these branches Alice falls into the hole, which subsequently evaporates. On the other branch Alice does not fall into the hole and consequently is still present in $\Psi_f$.