Philosophy: What exactly is a black hole?

A black hole is conventionally thought of as an astronomical object that irrevocably consumes all matter and radiation which comes within its sphere of influence. Physically, a black hole is defined by the presence of a singularity, i.e., a region of space, bounded by an 'event horizon', within which the mass/energy density becomes infinite, and the normally well-behaved laws of physics no longer apply. However, as an article in the January issue of the journal Nature Astronomy demonstrates, a precise and agreed definition of this 'singular' state proves to be frustratingly elusive. Its author, Dr. Erik Curiel of the Munich Center for Mathematical Philosophy at Ludwig-Maximilians-Universitaet, summarizes the problem as follows: "The properties of black holes are the subject of investigations in a range of subdisciplines of physics -- in optical physics, in quantum physics and of course in astrophysics. But each of these specialties approaches the problem with its own specific set of theoretical concepts."