Firewall or smooth horizon?

TL;DR

AMPS argued that maintaining horizon regularity with the complementarity postulates leads to a firewall for old BHs. This paper adopts a conservative semiclassicality postulate, asserting that semiclassical gravity holds as long as curvature is well below the Planck scale, which preserves horizon regularity and implies that most information remains inside the shrinking BH during the semiclassical phase. The author supports this with a hard-disc thought experiment and a geometric analysis showing the interior volume behind a narrow throat remains vast, so the BH information capacity scales with the initial mass $M_0$ rather than the instantaneous mass $m$ while Hawking radiation carries little information. Consequently, no firewall is required; the information's final fate could be a long-lived remnant that slowly releases information, or other remnant/baby-universe scenarios, challenging the dominant prompt-information-release view and urging reconciliation with semiclassical gravity while noting potential AdS/CFT tensions.

Abstract

Recently, Almheiri, Marolf, Polchinski, and Sully found that for a sufficiently old black hole (BH), the set of assumptions known as the \emph{complementarity postulates} appears to be inconsistent with the assumption of local regularity at the horizon. They concluded that the horizon of an old BH is likely to be the locus of local irregularity, a "firewall". Here I point out that if one adopts a different assumption, namely that semiclassical physics holds throughout its anticipated domain of validity, then no inconsistency seems to arise, and the horizon retains its regularity. In this alternative view-point, the vast portion of the original BH information remains trapped inside the BH throughout the semiclassical domain of evaporation, and possibly leaks out later on. This appears to be an inevitable outcome of semiclassical gravity.

Figures (3)

• Figure 1: Spacetime diagram of an (uncharged) evaporating spherical BH. The horizontal red line denotes the $r=0$ singularity, the dashed blue line S is the worldline of the collapsing thin shell, and the bold diagonal black line marked by H is the event horizon. The point "p", the intersection of the horizon and the $r=0$ singularity, is the "end of evaporation" point, which is actually a (naked) singularity of the semiclassical spacetime. The diagonal dashed line marked by "(CH)" (which extends the horizon in the upper-right direction) is in fact a Cauchy horizon of the semiclassical spacetime, beyond which the extension of geometry is not really predictable by the semiclassical theory. The green dashed line denotes the spacelike hypersurface $\Sigma$ (see text). The points marked by c, x, and m respectively denote the intersection points of $\Sigma$ with the regular center, with the collapsing shell, and with the horizon (at a "moment" $v$ where the remaining mass is $m$, much smaller than the initial mass $M_{0}$).
• Figure 2: The geometry of the spacelike hypersurface $\Sigma$, represented her by the corresponding function $r(L)$, where $r$ is the area coordinate and $L$ is the proper length along the hypersurface $\Sigma$ (in the radial direction). The points c, x, and m (see caption of Fig. 1) are marked.
• Figure 3: Embedding diagram for the hypersurface $\Sigma$. Note the narrow throat (at the horizon-crossing point "m") and the large internal volume.