DYNAMICAL ORIGIN OF THE ENTROPY OF A BLACK-HOLE

Modes of physical fields which are located inside a horizon and which cannot be observed by a distant observer are identified with the dynam ical degrees of freedom of a black hole. A new invariant statistical m echanical definition of black-hole entropy is proposed. It is shown th at the main contribution to the entropy is given by thermally excited ''invisible'' modes propagating in the close vicinity of the horizon. A calculation based on the proposed definition yields a value of the e ntropy which is in good agreement with the usually adopted value A(H)/ (4l(Pl)2), where A(H) is the black-hole surface area and l(Pl) is the Planck length.