When black holes collide: Probing the quantum state of black holes by the spectrum of emitted gravitational waves

Black hole (BH) mergers can be viewed as cosmological ``scattering experiments'' resulting in an excited BH, which then decays to its ground state by emitting gravitational waves. I will present general arguments, based on fundamental physics principles, as to why we should expect additional quantum ringdown modes to those predicted by general relativity. Then, I will discuss a model of BHs as bound states of fundamentals strings in the Hagedorn phase and the spectrum of the additional quasi-normal modes. Finally, I will discuss the spectrum of the predicted ringdown modes; the resulting emitted gravitational waves and the prospects for their detection in LIGO/VIRGO.