A Primer for Black Hole Quantum Physics

TL;DR

This paper reviews the mechanisms of Hawking radiation, analyzing pair production near horizons, and discusses the semiclassical approach's limitations, connecting black hole evaporation to thermodynamics and highlighting open issues at the quantum gravity frontier.

Contribution

It provides a detailed analysis of pair production and Hawking radiation, emphasizing the semiclassical theory's drawbacks and its relation to black hole thermodynamics and quantum gravity.

Findings

Hawking radiation arises from mode singularities at the horizon.

Semiclassical theory predicts black hole mass loss proportional to 1/M^2.

Fluctuations in field configurations reveal issues with unitarity and energy scales.

Abstract

The mechanisms which give rise to Hawking radiation are revealed by analyzing in detail pair production in the presence of horizons. In preparation for the black hole problem, three preparatory problems are dwelt with at length: pair production in an external electric field, thermalization of a uniformly accelerated detector and accelerated mirrors. In the light of these examples, the black hole evaporation problem is then presented. The leitmotif is the singular behavior of modes on the horizon which gives rise to a steady rate of production. Special emphasis is put on how each produced particle contributes to the mean albeit arising from a particular vacuum fluctuation. It is the mean which drives the semiclassical back reaction. This aspect is analyzed in more detail than heretofore and in particular its drawbacks are emphasized. It is the semiclassical theory which gives rise to Hawking's famous equation for the loss of mass of the black hole due to evaporation $dM/dt \simeq -1/M^2$. Black hole thermodynamics is derived from the evaporation process whereupon the reservoir character of the black hole is manifest. The relation to the thermodynamics of the eternal black hole through the Hartle--Hawking vacuum and the Killing identity are displayed. It is through the analysis of the fluctuations of the field configurations which give rise to a particular Hawking photon that the dubious character of the semiclassical theory is manifest. The present frontier of research revolves around this problem and is principally concerned with the fact that one calls upon energy scales that are greater than Planckian and the possibility of a non unitary evolution as well. These last subjects are presented in qualitative fashion only, so that this review stops at the threshold of quantum gravity.