  Null surfaces act as one-way membranes, blocking information from those
observers who do not cross them (e.g., in the black hole and the Rindler
spacetimes) and these observers associate an entropy (and temperature) with the
null surface. The black hole entropy can be computed from the central charge of
an appropriately defined, local, Virasoro algebra on the horizon. We show that
one can extend these ideas to a general class of null surfaces, all of which
possess a Virasoro algebra and a central charge, leading to an entropy density
(i.e., per unit area) which is just $(1/4)$. All the previously known results
of associating entropy with horizons arise as special cases of this very
general property of null surfaces demonstrated here and we believe this work
represents the derivation of the entropy-area law in the most general context.
The implications are discussed.
