  The Bekenstein bound takes the holographic principle into the realm of flat
space, promising new insights on the relation of non-gravitational physics to
quantum gravity. This makes it important to obtain a precise formulation of the
bound. Conventionally, one specifies two macroscopic quantities, mass and
spatial width, which cannot be simultaneously diagonalized. Thus, the counting
of compatible states is not sharply defined. The resolution of this and other
formal difficulties leads naturally to a definition in terms of discretized
light-cone quantization. In this form, the area difference specified in the
covariant bound converts to a single quantum number, the harmonic resolution K.
The Bekenstein bound then states that the Fock space sector with K units of
longitudinal momentum contains no more than exp(2 pi^2 K) independent discrete
states. This conjecture can be tested unambiguously for a given Lagrangian, and
it appears to hold true for realistic field theories, including models arising
from string compactifications. For large K, it makes contact with more
conventional but less well-defined formulations.
