The stretched horizon and black hole complementarity

Three postulates asserting the validity of conventional quantum theory, semiclassical general relativity, and the statistical basis for thermodynamics are introduced as a foundation for the study of black-hole evolution. We explain how these postulates may be implemented in a "stretched horizon" or membrane description of the black hole, appropriate to a distant observer. The technical analysis is illustrated in the simplified context of (1+1)-dimensional dilaton gravity. Our postulates imply that the dissipative properties of the stretched horizon arise from a course graining of microphysical degrees of freedom that the horizon must possess. A principle of black-hole complementarity is advocated. The overall viewpiont is similar to that poineered by 't Hooft but the detailed implementation is different.