Over the horizon: Distinguishing the Schwarzschild spacetime and the RP3 spacetime using an Unruh-DeWitt detector

The RP3 geon and the Schwarzschild black hole are two black hole spacetimes which differ only behind the event horizon. We show that the thermal Hawking radiation emanating from the two black holes contains nontrivial correlations, that these correlations contain information about their interiors, and demonstrate that a particle detector can recover these correlations. In this manner, a simple particle detector can determine the structure behind the event horizon of an eternal black hole.