[Paper Review] Do Black Holes Destroy Information?

John Preskill|CaltechAUTHORS (California Institute of Technology)|Sep 16, 1992

Cosmology and Gravitation Theories79 citations

TL;DR

This paper examines the black hole information paradox, arguing that if Hawking radiation is truly thermal, quantum information is lost during black hole evaporation—contradicting unitarity in quantum mechanics. The author contends this paradox may signal a fundamental crisis in physics, necessitating a radical revision of quantum theory or general relativity, and explores alternatives like information retention in radiation or stable remnants.

ABSTRACT

I review the information loss paradox that was first formulated by Hawking, and discuss possible ways of resolving it. All proposed solutions have serious drawbacks. I conclude that the information loss paradox may well presage a revolution in fundamental physics. (To appear in the proceedings of the International Symposium on Black Holes, Membranes, Wormholes, and Superstrings.)

Motivation & Objective

• To analyze the information loss paradox arising from Hawking's semiclassical calculation of black hole radiation.
• To evaluate proposed resolutions to the paradox, including information retention in Hawking radiation and the existence of stable black hole remnants.
• To assess whether the breakdown of quantum unitarity in black hole evaporation implies a need for a new fundamental theory of physics.
• To explore scattering processes off extreme black holes as a probe of information preservation without requiring full quantum gravity.
• To investigate the feasibility of phenomenological models of information loss that preserve low-energy physics while allowing for coherence breakdown at high energies.

Proposed method

• Analyzes the semiclassical approximation of Hawking radiation, where gravitational back-reaction is neglected, leading to thermal emission independent of initial state.
• Considers the S-matrix formalism for scattering off extreme black holes, which are stable and do not emit radiation, to test unitarity.
• Reviews the (1+1)-dimensional effective field theory approach in dilaton gravity, which includes back-reaction and allows for analytical study of particle scattering.
• Evaluates the consistency of models where information is encoded in outgoing radiation, using causality and unitarity as constraints.
• Examines the implications of non-unitary evolution, such as density matrix evolution, and critiques proposals that introduce Planck-scale noise to explain decoherence.
• Assesses phenomenological limits on information loss, focusing on energy conservation and locality constraints in modified quantum theories.

Experimental results

Research questions

• RQ1 Can information be preserved in Hawking radiation despite the absence of 'hair' on black holes?
• RQ2 Is the assumption of thermal radiation in Hawking's calculation sufficient to conclude that information is lost?
• RQ3 Do extreme black holes, which are classically stable, allow for a unitary S-matrix description of scattering, thereby testing information conservation?
• RQ4 Can a consistent phenomenological theory of information loss be constructed that preserves low-energy quantum field theory while allowing for non-unitarity at high energies?
• RQ5 What are the physical and conceptual consequences of abandoning unitarity in quantum mechanics due to black hole evaporation?

Key findings

• The paradox arises because a pure quantum state evolving into a mixed state via black hole evaporation violates unitarity, a cornerstone of quantum mechanics.
• Hawking's original calculation, based on semiclassical gravity, predicts thermal radiation independent of the initial state, suggesting information loss.
• The absence of correlations between outgoing radiation and the initial state is enforced by causality, as no signals can escape from behind the event horizon.
• The hypothesis that information is encoded in Hawking radiation remains viable but lacks a concrete mechanism, especially in the absence of back-reaction effects.
• The existence of stable black hole remnants is a conservative resolution, but faces strong objections, including infinite degeneracy and energy conservation violations.
• Scattering off extreme black holes in (1+1)-dimensional dilaton gravity provides a tractable model to probe information preservation, though full analysis exceeds semiclassical validity.