  We consider the radial geodesic motion of a massive particle into a black
hole in isotropic coordinates, which represents the exterior region of an
Einstein-Rosen bridge (wormhole). The particle enters the interior region,
which is regular and physically equivalent to the asymptotically flat exterior
of a white hole, and the particle's proper time extends to infinity. Since the
radial motion into a wormhole after passing the event horizon is physically
different from the motion into a Schwarzschild black hole, Einstein-Rosen and
Schwarzschild black holes are different, physical realizations of general
relativity. Yet for distant observers, both solutions are indistinguishable. We
show that timelike geodesics in the field of a wormhole are complete because
the expansion scalar in the Raychaudhuri equation has a discontinuity at the
horizon, and because the Einstein-Rosen bridge is represented by the Kruskal
diagram with Rindler's elliptic identification of the two antipodal future
event horizons. These results suggest that observed astrophysical black holes
may be Einstein-Rosen bridges, each with a new universe inside that formed
simultaneously with the black hole. Accordingly, our own Universe may be the
interior of a black hole existing inside another universe.
