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We propose an S matrix approach to the quantum black hole in which causality,unitarity and their interrelation play a prominent role. Assuming the 't HooftS matrix ansatz for a gravitating region surrounded by an asymptotically flatspace-time we find a non-local transformation which changes the standardcausality requirement but is a symmetry of the unitarity condition of the Smatrix. This new S matrix then implies correlations between the in and outstates of the theory with the involvement of a third entity which in the caseof a quantum black hole, we argue is the horizon S matrix. Such correlationsare thus linked to preserving the unitarity of the S matrix and to the factthat entangling unitary operators are nonlocal. The analysis is performedwithin the Bogoliubov S matrix framework by considering a spacetime consistingof causal complements with a boundary in between. No particular metric orlagrangian dynamics need be invoked even to obtain an evolution equation forthe full S matrix. Constraints imposed by the new causality requirement andimplications for the effectiveness of field theoretical descriptions and forcomplementarity are also discussed. We find that the tension betweeninformation preservation and complementarity may be resolved provided the fullquantum gravity theory either through symmetries or fine tuning forbids theoccurrence of closed time like curves of information flow. Then, even ifcausality is violated near the horizon at any intermediate stage, a standardcausal ordering may be preserved for the observer away from the horizon. In thecontext of the black hole, the novelty of our formulation is that it appearswell suited to understand unitarity at any intermediate stage of black holeevaporation. Moreover, it is applicable generally to all theories with longrange correlations including the final state projection models.

Unitary S Matrices With Long-Range Correlations and the Quantum Black Hole