Exponential Corrections to Black Hole Entropy

Using the quasilocal properties alone we show that the area spectrum of a black hole horizon must be discrete, independent of any specific quantum theory of gravity. The area spectrum is found to be half-integer spaced with values 8 pi gamma l(p)(2)j where j is an element of N/2. We argue that if microstate counting is carried out for quantum states residing on the horizon only, correction of exp(-A/4l(p)(2)) over the Bekenstein-Hawking area law must arise in black hole entropy.