To summarize, black holes are the most universal localized objects in any theory that includes general relativity. They’re not just random siblings of comets, planets, stars, and quasars: they’re special. They maximize the density of many things. Despite the large matter densities that are needed to produce them, black holes are connected with “apparently empty” space, space without matter. Their properties therefore have something to do with the properties of the empty space itself.
— Event horizons and thermodynamics: more than an analogy
— Lubos Motl
2013.05.04 Saturday ACHK