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Naively, the information content of Nature would seem to depend on the
size of the smallest constituents and should scale with the volume of
regions. However, there is strong evidence that it is set instead by the
area of surfaces. I will review the developments that led to the
recognition of this entropy bound, giving special attention to the quantum
properties of black holes. I will explain how to construct "light-sheets",
which define the regions whose entropy is associated to any given surface
in the universe. The empirical pattern thus uncovered suggests a general
relation between geometry and information. Understanding its origin would
likely provide us with a key insight into quantum gravity.
hep-th/0203101
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