Stephen Hawking's discovery of black hole evaporation has the remarkable consequence that information is destroyed by a black hole, which can only be accommodated by modifying the laws of quantum mechanics. Different attempts to evade the information loss paradox were subsequently suggested, apparently without a satisfactory resolution of the paradox. On the other hand, attempting to include nonunitarity in quantum mechanics might lead to laws predicting observable consequences such as nonlocality or violation of energy–momentum conservation; but it may be possible to circumvent these obstacles. Recent developments seem to require a different view on quantum gravity and suggest that ideas about locality in physics and Hawking's semiclassical approximation are misleading. An accurate description may show unitary evolution and no information loss after all.
Stephen Hawking's discovery of black hole evaporation has the remarkable consequence that information is destroyed by a black hole, which can only be accommodated by modifying the laws of quantum mechanics. Different attempts to evade the information loss paradox were subsequently suggested, apparently without a satisfactory resolution of the paradox. On the other hand, attempting to include nonunitarity in quantum mechanics might lead to laws predicting observable consequences such as nonlocality or violation of energy–momentum conservation; but it may be possible to circumvent these obstacles. Recent developments seem to require a different view on quantum gravity and suggest that ideas about locality in physics and Hawking's semiclassical approximation are misleading. An accurate description may show unitary evolution and no information loss after all.
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.
Black hole evaporation is one of the most striking phenomena at the interface between gravity and quantum physics. In Hawking's semi-classical treatment, where matter is quantum mechanical and the spacetime is definite and classical, evaporation leads to an apparent loss of unitarity of the overall evolution, and to the so-called black hole information paradox. Here, we go beyond this semi-classical treatment and formulate a toy quantum model of black hole evaporation that allows the black hole to evolve into a superposition of being fully evaporated and not fully evaporated, consistent with the Hawking particles being in a coherent superposition of different energy levels. We model Hawking particle production by the repeated action of quantum-controlled unitaries, generating emission from the quantum black hole and accounting for a quantum coherent back-reaction on the black hole matter state. We show that the probability of full annihilation of the black hole matter increases with time until the black hole is, asymptotically, fully evaporated in every branch of the quantum superposition. We prove that under natural assumptions, this evaporation model is unitary, such that the initial state can in principle be recovered from the final asymptotic state of the radiation.
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.
We consider a novel approach to address the black hole information paradox. The idea is based on adapting, to the situation at hand, the modified versions of quantum theory involving spontaneous stochastic dynamical collapse of quantum states, which have been considered in attempts to deal with shortcomings of the standard Copenhagen interpretation of quantum mechanics, in particular, the issue known as ``the measurement problem.'' The new basic hypothesis is that the modified quantum behavior is enhanced in the region of high curvature so that the information encoded in the initial quantum state of the matter fields is rapidly erased as the black hole singularity is approached. We show that in this manner the complete evaporation of the black hole via Hawking radiation can be understood as involving no paradox. Calculations are performed using a modified version of quantum theory known as ``continuous spontaneous localization'' (CSL), which was originally developed in the context of many-particle nonrelativistic quantum mechanics. We use a version of CSL tailored to quantum field theory and applied in the context of the two -dimensional Callan-Giddings-Harvey-Strominger model. Although the role of quantum gravity in this picture is restricted to the resolution of the singularity, related studies suggest that there might be further connections.
The black hole information paradox remains one of modern physics’ most persistent conceptual fractures. Hawking’s original formulation suggested that black hole evaporation yields thermal radiation, apparently severing the relation between the initial state and the final emitted state. Quantum theory, however, requires unitary evolution and therefore forbids true information destruction. This paper proposes that the paradox may be more fruitfully interpreted not as a flat binary contradiction between preservation and loss, but as an unresolved boundary-state problem. To clarify that claim, the paper introduces the 101 Phase State as a symbolic formalism for intermediate informational resolution. In this model, +1 denotes preserved or recoverable informational order, -1 denotes apparent loss or thermal unreadability, and 0 denotes the boundary interface at which observational contradiction becomes structurally legible but not yet fully resolved.
Suppose a black hole forms from a pure quantum state ∣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 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.