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Black Holes: Complementarity or Firewalls?

Ahmed Almheiri, Donald Marolf, Joseph Polchinski, James Sully

TL;DR

The paper investigates the black hole information paradox by examining whether unitarity, semiclassical outside-horizon physics, and a drama-free infalling experience can coexist. Through thought experiments using Hawking radiation and extensions to higher partial waves, it argues that at least one of these assumptions must fail, pointing to a firewall or to macroscopic nonlocal dynamics beyond the horizon. It further analyzes the possibility of relaxing semiclassical postulates and finds that such a route would require strong nonlocal effects incompatible with effective field theory. The work thus strengthens the case for dramatic horizon behavior or far-reaching new physics to resolve how information escapes a black hole. This challenges conventional pictures of smooth horizons and local EFT in gravitational collapse and has broad implications for holography and quantum gravity.

Abstract

We argue that the following three statements cannot all be true: (i) Hawking radiation is in a pure state, (ii) the information carried by the radiation is emitted from the region near the horizon, with low energy effective field theory valid beyond some microscopic distance from the horizon, and (iii) the infalling observer encounters nothing unusual at the horizon. Perhaps the most conservative resolution is that the infalling observer burns up at the horizon. Alternatives would seem to require novel dynamics that nevertheless cause notable violations of semiclassical physics at macroscopic distances from the horizon.

Black Holes: Complementarity or Firewalls?

TL;DR

The paper investigates the black hole information paradox by examining whether unitarity, semiclassical outside-horizon physics, and a drama-free infalling experience can coexist. Through thought experiments using Hawking radiation and extensions to higher partial waves, it argues that at least one of these assumptions must fail, pointing to a firewall or to macroscopic nonlocal dynamics beyond the horizon. It further analyzes the possibility of relaxing semiclassical postulates and finds that such a route would require strong nonlocal effects incompatible with effective field theory. The work thus strengthens the case for dramatic horizon behavior or far-reaching new physics to resolve how information escapes a black hole. This challenges conventional pictures of smooth horizons and local EFT in gravitational collapse and has broad implications for holography and quantum gravity.

Abstract

We argue that the following three statements cannot all be true: (i) Hawking radiation is in a pure state, (ii) the information carried by the radiation is emitted from the region near the horizon, with low energy effective field theory valid beyond some microscopic distance from the horizon, and (iii) the infalling observer encounters nothing unusual at the horizon. Perhaps the most conservative resolution is that the infalling observer burns up at the horizon. Alternatives would seem to require novel dynamics that nevertheless cause notable violations of semiclassical physics at macroscopic distances from the horizon.

Paper Structure

This paper contains 9 sections, 24 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Complementarity is not enough
  3. Further discussion
  4. Extension to higher partial waves
  5. Relaxing postulate 2?
  6. Conclusions
  7. Approximate projection operators
  8. Effects of Gray-body factors
  9. Black hole mining revisited

Figures (1)

  • Figure 1: Eddington-Finkelstein coordinates, showing the infalling observer encountering the outgoing Hawking mode (shaded) at a time when its size is $\omega_*^{-1} \ll r_{\rm s}$. If the observer's measurements are given by an eigenstate of $a^\dagger a$, postulate 1 is violated; if they are given by an eigenstate of $b^\dagger b$, postulate 4 is violated; if the result depends on when the observer falls in, postulate 2 is violated.