Black holes without firewalls

TL;DR

This paper questions the claim that black hole complementarity necessitates a firewall and argues that a local, causal stretched-horizon dynamics with information retention on a scrambling-time scale $t_{scramble} \sim M \log M$ can reconcile unitarity with a smooth horizon. It develops a Hayden-Preskill–style framework distinguishing long- and short-wavelength Hawking emissions, showing information can emerge outside the horizon without forcing drama for infalling observers. It also analyzes entropy subadditivity in stretched-horizon EFT and global-horizon models, finding no universal violation. The result supports black hole complementarity and clarifies that firewall-like behavior would require interior-dynamics assumptions beyond the standard postulates.

Abstract

The postulates of black hole complementarity do not imply a firewall for infalling observers at a black hole horizon. The dynamics of the stretched horizon, that scrambles and re-emits information, determines whether infalling observers experience anything out of the ordinary when entering a large black hole. In particular, there is no firewall if the stretched horizon degrees of freedom retain information for a time of order the black hole scrambling time.

Figures (4)

• Figure 1: Penrose diagram for black hole evaporation. $\Sigma_{0}$ is the global horizon and $\Sigma$ is a stretched horizon.
• Figure 2: Different infalling observers encountering outgoing Hawking modes. The stretched horizon is shown as the right dashed line, and the global horizon as the left dashed line. Before the infalling diary scrambles on the stretched horizon, the outgoing mode is unentangled with it. Only after scrambling will an infalling observer notice entanglement with the diary. Proper time along the stretched horizon provides a distinguished set of clocks which demarcate this interval.
• Figure 3: The analog of figure \ref{['fig:Different-infalling-observers']} with the diary replaced the ordinary Hawking modes. An infaller measuring a mode outside the stretched horizon (infaller 2) will see it maximally entangled with the early Hawking radiation. However an earlier infalling observer (infaller 1) must see vanishing entanglement with the early Hawking radiation if Postulate 4 holds.
• Figure 4: The Penrose diagram for the maximally extended Schwarzschild black hole. The area to the right of the dashed line provides a classical model for black hole formation. The Hilbert space of states may be factored into states on the interior and the exterior along the time-slice indicated by the horizontal line. Both sets of modes propagate at later times into the upper quadrant.