Quantum state of the black hole interior

TL;DR

This paper models the black hole interior as a highly quantum state resembling a Fermi sea, which decays via Hawking radiation and causes infalling objects to disintegrate rapidly after crossing the horizon, challenging classical notions.

Contribution

It introduces a novel quantum state model of the black hole interior as a dense collection of excitations, providing insights into black hole evaporation and the fate of infalling objects.

Findings

Black hole interior modeled as a Fermi sea of excitations.

Decay rate of the quantum state matches Hawking radiation.

Infalling objects disintegrate rapidly after crossing the horizon.

Abstract

If a black hole (BH) is initially in an approximately pure state and it evaporates by a unitary process, then the emitted radiation will be in a highly quantum state. As the purifier of this radiation, the state of the BH interior must also be in some highly quantum state. So that, within the interior region, the mean-field approximation cannot be valid and the state of the BH cannot be described by some semiclassical metric. On this basis, we model the state of the BH interior as a collection of a large number of excitations that are packed into closely spaced but single-occupancy energy levels; a sort-of "Fermi sea" of all light-enough particles. This highly quantum state is surrounded by a semiclassical region that lies close to the horizon and has a non-vanishing energy density. It is shown that such a state looks like a BH from the outside and decays via gravitational pair production in the near-horizon region at a rate that agrees with the Hawking rate. We also consider the fate of a classical object that has passed through to the BH interior and show that, once it has crossed over the near-horizon threshold, the object meets its demise extremely fast. This result cannot be attributed to a "firewall", as the trauma to the in-falling object only begins after it has passed through the near-horizon region and enters a region where semiclassical spacetime ends but the energy density is still parametrically smaller than Planckian.