Holographic entropy bound from gravitational Fock space truncation

A simplified derivation of Yurtsever's result, which states that the entropy of a truncated bosonic Fock space is given by a holographic bound when the energy of the Fock states is constrained gravitationally, is given for asymptotically flat spacetimes with arbitrary dimension d greater or equal to four. For this purpose, a scalar field confined to a spherical volume in d-dimensional spacetime is considered. Imposing an upper bound on the total energy of the corresponding Fock states which ensures that the system is in a stable configuration against gravitational collapse and imposing a cutoff on the maximum energy of the field modes of the order of the Planck energy leads to an entropy bound of holographic type. A simple derivation of the entropy bound is also given for the fermionic case.