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Germani, Cristiano; Sarkar, Debajyoti (2016): Firewalls as artefacts of inconsistent truncations of quantum geometries. In: Fortschritte der Physik - Progress of Physics, Vol. 64, No. 1: pp. 131-143
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In this paper we argue that a firewall is simply a manifestation of an inconsistent truncation of non-perturbative effects that unitarize the semiclassical black hole. Namely, we show that a naive truncation of quantum corrections to the Hawking spectrum at order O(e(-S)), inexorably leads to a "localised" divergent energy density near the black hole horizon. Nevertheless, in the same approximation, a distant observer only sees a discretised spectrum and concludes that unitarity is achieved by O(e(-S)) effects. This is due to the fact that instead, the correct quantum corrections to the Hawking spectrum go like O(g(tt)e(-S)). Therefore, while at a distance far away from the horizon, where g(tt) approximate to 1, quantum corrections are perturbative, they do diverge close to the horizon, where g(tt) -> infinity. Nevertheless, these "corrections" nicely re-sum so that correlations functions are smooth at the would-be black hole horizon. Thus, we conclude that the appearance of firewalls is just a signal of the breaking of the semiclassical approximation at the Page time, even for large black holes.