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Physics Today

Can information get back out of black holes?

Published May 23, 2008• 0 citations
Publisher

Abstract

Nature News: If you were sucked into a black hole, you wouldn't stand a chance. But new calculations suggest that some things might survive travelling to the heart of the Universe's darkest objects. 'Quantum information' could make it through a black hole, says a group of theorists at Pennsylvania State University. If their calculation holds water, it would solve an important problem for quantum mechanics âmdash; and make the behaviour of black holes easier to predict.

DOI
10.1063/pt.5.022292

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Nature News: If you were sucked into a black hole, you wouldn't stand a chance. But new calculations suggest that some things might survive travelling to the heart of the Universe's darkest objects. 'Quantum information' could make it through a black hole, says a group of theorists at Pennsylvania State University. If their calculation holds water, it would solve an important problem for quantum mechanics âmdash; and make the behaviour of black holes easier to predict.
Abstract ‘Black Holes and the Edge of Physics’ delves into the mysteries of black holes, and shows how these enigmatic objects may transcend quantum physics. We begin with a review of how black holes form and eventually evaporate, resulting in the notorious problem of ‘information loss’. We show how pilot-wave theory could solve the problem by allowing information to escape at superluminal speeds. We describe an experiment that could test these and related ideas, involving astrophysical black holes in space. The chapter ends with an account of the life cycle of black holes and the role they may play in ‘reprocessing’ our quantum universe.
Suppose a black hole forms from a pure quantum state ∣ψ⟩\ketψ∣ψ⟩. The black hole information loss paradox arises from semiclassical arguments suggesting that, even in a closed system, the process of black hole formation and evaporation evolves a pure state into a mixed state. Resolution to the paradox typically demands violation of quantum mechanics or relativity in domains where they should hold. Instead, I propose that in a complete theory of quantum gravity, any region U\mathcal{U}U that could collapse into a black hole should already be described by a mixed state, thus bypassing the paradox entirely. To that end, I present a model in which the universe is in a quantum error-corrected state, such that any local black hole appears mixed and encodes no information locally.
Thirty years ago, John Preskill concluded "that the information loss paradox may well presage a revolution in fundamental physics" and mused that "Conceivably, the puzzle of black hole evaporation portends a scientific revolution as sweeping as that that led to the formulation of quantum theory in the early 20th century." Many still agree with this assessment. On the other hand, it seems to me the "paradox" has little to do with the physical world but rather, at best, simply points out the possible inconsistency of two, already disparate, theories (mathematical models) of nature, general relativity and quantum mechanics, with virtually no conceivable observational consequences. The information paradox hinges on the concepts of a pure quantum state, the unitarity of quantum mechanics, and Hawking's semi-classical calculation of black hole evaporation. I used the qualifier "at best" above because, for me, the concept of a quantum state is far more restrictive than required by the paradox while unitarity is not a property of nature but rather of a mathematical model and is already violated by the process of making a measurement. Furthermore, the semi-classical calculation of Hawking is surely of limited applicability.
Can quantum-information theory shed light on black-hole evaporation? By entangling the in-fallen matter with an external system we show that the black-hole information paradox becomes more severe, even for cosmologically sized black holes. We rule out the possibility that the information about the in-fallen matter might hide in correlations between the Hawking radiation and the internal states of the black hole. As a consequence, either unitarity or Hawking's semiclassical predictions must break down. Any resolution of the black-hole information crisis must elucidate one of these possibilities.