  It was shown recently that, in the case of Schwarschild black hole, one can
obtain the correct thermodynamic relations by studying a model quantum system
and using a particular duality transformation. We study this approach further
for the case a general spherically symmetric horizon. We show that the idea
works for a general case only if we define the entropy S as a congruence
("observer") dependent quantity and the energy E as the integral over the
source of the gravitational acceleration for the congruence. In fact, in this
case, one recovers the relation S=E/2T between entropy, energy and temperature
previously proposed by one of us in gr-qc/0308070. This approach also enables
us to calculate the quantum corrections of the Bekenstein-Hawking entropy
formula for all spherically symmetric horizons.
