  We consider a spacelike two-plane originally at rest with respect to
electromagnetic radiation in equilibrium. We find that if the plane is moved
with respect to the radiation, the plane shrinks such that the maximum amount
of entropy carried by radiation through the plane is, in natural units, exactly
one-half of the decrease in the area of the plane. This result suggests that
the equivalence between area and entropy may not be limited in black holes, nor
even in the spacetime horizons only, but the equivalence between horizon area
and entropy may be a special case of some general and simple, still
undiscovered fundamental principle of nature.

General Relativity and Quantum Cosmology