  Spherical configurations that are very massive must be surrounded by apparent
horizons. These in turn, when placed outside a collapsing body, have a fixed
area and must propagate outward with a velocity equal to the velocity of
radially outgoing photons. That proves, within the framework of the (1+3)
formalism and without resorting to the Birkhoff theorem, that apparent horizons
coincide with event horizons in electrovacuum.
