Casimir-Polder-like force on an atom outside a Schwarzschild black hole

We calculate, in the framework of open quantum systems, the ground state energy-level shift for a static two-level atom outside a spherically symmetric black hole in interaction with fluctuating massless scalar fields in the Boulware and Unruh vacuums. We find that the energy-level shift is position-dependent and thus gives rise to a force on the atom besides the classical gravitational force. For the case of the Boulware vacuum which represents a star that has not collapsed through its event horizon, this force is attractive near the horizon and is repulsive far away from the black hole with a behavior of $r^{-3}$ . For the case of the Unruh vacuum which represents a radiating black hole, we find that the contribution to the Casimir-Polder-like force due to the presence of Hawking radiation is always attractive and, remarkably, this attractive force diverges at the event horizon.