Non-modal effects: Transient Superradiance

Black hole quasinormal modes are governed by a non-normal evolution operator with respect to a natural choice of inner product, and thus do not form a complete and orthogonal basis. This allows linear perturbations around a black hole spacetime to potentially exhibit large-amplitude growth at finite time, despite asymptotic decay at later times. In this talk, I will exemplify this mechanism by discussing a transient form of superradiance, drawing similarities to the transition to turbulence in Navier-Stokes shear flows.