The information paradox in the quantum evolution of black holes is studied
within the framework of the anti-de Sitter-conformal field theory (AdS-CFT)
correspondence. The unitarity of the CFT strongly suggests that all inform
ation about an initial state that forms a black hole is returned in the Haw
king radiation. The CFT dynamics implies an information retention time of t
he order of the black-hole lifetime. This fact determines many qualitative
properties of the nonlocal effects that must show up in a semiclassical eff
ective theory in the bulk. We argue that no Violations of causality are app
arent to local observers, but the semiclassical theory in the bulk duplicat
es degrees of freedom inside and outside the event horizon. Nonlocal quantu
m effects are required to eliminate this redundancy. This leads to a breakd
own of the usual classical-quantum correspondence principle in Lorentzian b
lack-hole spacetimes. [S0556-2821(99)02618-1].