  The maximum entropy that can be stored in a bounded region of space is in
dispute: it goes as volume, implies (non-gravitational) microphysics; it goes
as the surface area, asserts the "holographic principle." Here I show how the
holographic bound can be derived from elementary flat-spacetime quantum field
theory when the total energy of Fock states is constrained gravitationally.
This energy constraint makes the Fock space dimension (whose logarithm is the
maximum entropy) finite for both Bosons and Fermions. Despite the elementary
nature of my analysis, it results in an upper limit on entropy in remarkable
agreement with the holographic bound.
