INTRODUCTION

The internal workings of a black hole constitutes a relatively new field, still in its first faltering steps. These pages are intended as a brief introduction and progress report on what I think has been gleaned so far.

As far as external appearances go, black holes are matchlessly simple objects. I have described elsewhere (Israel 1987; Thorne 1993) how Charles Misner was probably the first to fully appreciate this. In the wake of a gravitational collapse, quadrupole and other deformations originally anchored in the star get swallowed by the hole or carried off by gravitational radiation. The external field and event horizon settle, like a newly-formed soap bubble, into the simplest configuration that is compatible with the external constraints—mass, charge and angular momentum. (The soap-bubble analogy, I believe, stems also from Misner.)

The immaculate exterior of a black hole hides an egregious inner disorder. A solarmass black hole holds 10 times as much entropy as a ball of radiation of the same mass-energy and volume.

The key to this dichotomy is the peculiar causal structure of the hole. The event horizon not only encloses but precedes the inner core where curvatures approach Planck levels. The surface of a black hole, unlike the surface of a star, is not influenced by processes in the core.