ABSTRACT
Black holes are solutions of Einstein’s equations in vacuum. Since these equations do not contain dimensionful parameters the mass, which arises as a dimensionful integration constant, must be a free parameter. This chapter shows that the equation identifies two black hole horizons of the Kerr metric and discusses their structure. Numerical simulations of astrophysical processes leading to black hole formation provide strong support to the hypothesis. In the case of Kerr’s geometry, the spacetime cannot be decomposed in the product of two-dimensional manifolds, thus the study of null geodesics is more complex than in the Schwarzschild case. The fascinating structure is unlikely to be realized in actual astrophysical objects.
