  We reformulate Holographic Space-time (HST) Models as Hilbert bundles over
the space of time-like geodesics on a background manifold. The background,
following Jacobson, is viewed as a hydrodynamic flow, which the quantum model
must reproduce. Work of many authors, but particularly the Verlindes, Carlip
and Solodukhin, suggest that the relevant quantum model is a sequence of 1+1
dimensional conformal field theories (CFT) on the boundaries of nested causal
diamonds. Finiteness of the entropy suggests the CFTs be built from cutoff
fermion fields, and the spin/statistics connection, combined with Connes'
demonstration that Riemannian geometry is encoded in the Dirac equation,
suggests that these fields are labelled by the cutoff eigenspectrum of the
Dirac operator on the holoscreen of each diamond. This leads to a natural
conjecture for the density matrix of arbitrary diamonds and,in a subclass of
space-times, for the time evolution operator between them. We conjecture that
the 't Hooft commutation relations on diamond boundaries are related to
Schwinger terms in U(1) currents constructed from the fermion fields. We review
the notion of "locality as constraints on holographic variables" discovered by
Fiol, Fischler and the present author and, in an appendix, explain how it
differs from the notion of locality arising from tensor network constructions
in AdS/CFT.
