 One of the most remarkable features of relativistic black holes is that they are purely gravitational entities: all the standard black-hole spacetime models (Schwarzschild, Reissner-Nordström, Kerr, Kerr-Newman) contain no matter whatsoever. They are vacuum solutions to the Einstein field equations, which just means a solution in which the matter density is everywhere zero. (Of course, one can also consider a black hole with matter present, as standard astrophysical models do for the supermassive black holes that are believed to live at the center of most galaxies, which are thought to be surrounded by strong magnetic fields and accretion disks of super-heated matter.) In pre-relativistic physics we think of gravity as a force produced by the mass associated with some matter. In the context of general relativity, however, we do away with gravitational force, and instead postulate a curved spacetime geometry that produces all the effects we standardly attribute to gravity. One of the most characteristic features of general relativity that sets it apart from Newtonian gravitational theory is that it admits the possibility of such curvature (“gravitational effects”) in the absence of matter, such as at the boundary of a black hole. Thus a black hole is not a thing in spacetime; it is instead a feature of spacetime itself. 