As derived in Sec. 2.1, the tunneling probability is zero for escape from inside a
single isolated black hole, or any isolated body. Let us see what effect an external
second body has on the tunneling probability out of a black hole of mass M. Let us take
a square well as the simplest approximation to a black hole. The tunneling probability
out of an isolated infinite square well is zero; and it may be harder to tunnel out of it
than an isolated black hole, as an infinite inward-directed force is encountered at its
surface. As shown in Fig. 2, we have a body of mass M2 centered at R2 opposite a black
hole centered at the origin of radius RH; and strength of the well V0RH ≈ GM2, where
V
0 is the potential energy depth of the well.