  We present a theory of accelerated observers in the formalism of holographic
space time, and show how to define the analog of the Unruh effect for a one
parameter set of accelerated observers in a causal diamond in Minkowski space.
The key fact is that the formalism splits the degrees of freedom in a large
causal diamond into particles and excitations on the horizon. The latter form a
large heat bath for the particles, and different Hamiltonians, describing a one
parameter family of accelerated trajectories, have different couplings to the
bath. We argue that for a large but finite causal diamond the Hamiltonian
describing a geodesic observer has a residual coupling to the bath and that the
effect of the bath is finite over the long time interval in the diamond. We
find general forms of the Hamiltonian, which guarantee that the horizon degrees
of freedom will decouple in the limit of large diamonds, leaving over a unitary
evolution operator for particles, with an asymptotically conserved energy. That
operator converges to the S-matrix in the infinite diamond limit. The S-matrix
thus arises from integrating out the horizon degrees of freedom, in a manner
reminiscent of, but distinct from, Matrix Theory. We note that this model for
the S-matrix implies that Quantum Gravity, as opposed to quantum field theory,
has a natural adiabatic switching off of the interactions. We argue that
imposing Lorentz invariance on the S-matrix is natural, and guarantees
super-Poincare invariance in the HST formalism. Spatial translation invariance
is seen to be the residuum of the consistency conditions of HST.
