Abstract

Once considered bizarre solutions to Einstein’s General Relativity, black holes have now acquired a central role in astrophysics, cosmology, particle and theoretical physics. Their physics connects gravity, thermodynamics and quantum mechanics, and the paradoxes they reveal are deep and far-reaching: their Hawking evaporation is incompatible with quantum unitarity, the microstates underlying their enormous entropy are unknown, and they conceal singularities where Einstein’s theory breaks down. One possibility, supported both by Quantum-Information-Theory arguments and by explicit String Theory constructions, is that these paradoxes are resolved by the existence of new horizon-scale physics. Gravitational-wave observations provide us with unprecedented probes of the nature of dark compact objects at the scale of the horizon. I will give an overview of the many recent results in this area (including ringdown tests, multipole moments, and tidal effects in binaries) and discuss the outstanding challenges ahead. These tests are to a large extent model agnostic, but tailored to and motivated by the string-theory fuzzball scenario.