Alternative characterization of a black hole boundary

After reviewing the shortcomings of existing definitions used to characterize the boundary of a black hole, we present a new method for its characterization. This definition could potentially be applied to locate the boundary of general black hole solutions of the Einstein field equations. Using our definition in general spherically symmetric spacetimes, we argue that an observer falling into a black hole encounters an infinite energy density, pressure, and flux at the black hole boundary. This result is interpreted as a firewall that prevents the growth of classical black holes. However, the possibility of black hole growth via quantum mechanical tunneling cannot be ruled out.