New Kind of Echo from Quantum Black Holes

Abstract

We propose that a quantum black hole can produce a new kind of late-time gravitational echoes, facilitated by a near-horizon process analogous to Andreev reflection in condensed matter systems. In comparison to the traditional echo scenarios where the near-horizon region is treated as an ordinary reflector, we argue that, consequent to near-horizon gravitational scattering, this region is better described by an Andreev reflector. Such interactions lead to a novel contribution to gravitational echoes with a characteristic phase difference, an effect which is analogous to how Andreev reflections lead to propagating particle-like and hole-like components with a relative phase in certain condensed matter scenarios. Moreover, this novel contribution to the echo signal encodes information about the 'near-horizon quantum state', hence offering a possible new window to probe the quantum nature of black holes.

Echoes from Classical Black Holes

“…Many ideals were put forward to create black hole echoes. Notable examples include quantum black holes [25][26][27][28], black hole with discontinuous effective potential [29], or with local Lorentz symmetry violations [30]. However, there is hitherto no example of classical black holes that can produce echoes.…”