Understand
In the corpuscular picture of black hole there exists no geometric notion of horizon which, instead, only emerges in the semi-classical limit.
- Therefore, it is very natural to ask - what happens if we send a signal towards a corpuscular black hole? We show that quantum effects at the horizon scale imply the existence of a surface located at an effective radius $R=R_s(1+\epsilon)$ slightly larger than the Schwarzschild radius $R_s,$ where $\epsilon=1/N$ and $N$ is the number of gravitons composing the system.
- Consequently, the reflectivity of the object can be non-zero and, indeed, we find that incoming waves with energies comparable to the Hawking temperature can have a probability of backscattering of order one.
- Thus, modes can be trapped between the two potential barriers located at the photon sphere and at the surface of a corpuscular black hole, and periodic echoes can be produced.