Quasinormal modes of AdS black holes and the approach to thermal equilibrium

We investigate the decay of a scalar field outside a Schwarzschild anti--de Sitter black hole. This is determined by computing the complex frequencies associated with quasinormal modes. There are qualitative differences from the asymptotically flat case, even in the limit of small black holes. In particular, for a given angular dependence, the decay is always exponential---there are no power law tails at late times. In terms of the AdS-CFT correspondence, a large black hole corresponds to an approximately thermal state in the field theory, and the decay of the scalar field corresponds to the decay of a perturbation of this state. Thus one obtains the time scale for the approach to thermal equilibrium. We compute these time scales for the strongly coupled field theories in three, four, and six dimensions, which are dual to string theory in asymptotically AdS spacetimes.