  We propose a new framework of quantum field theory for an arbitrary observer
in curved spacetime, defined in the spacetime region in which each point can
both receive a signal from and send a signal to the observer. Multiple
motivations for this proposal are discussed. We argue that radar time should be
applied to slice the observer's spacetime region into his simultaneity
surfaces. In the case where each such surface is a Cauchy surface, we construct
a unitary dynamics which evolves a given quantum state at a time for the
observer to a quantum state at a later time. We speculate on possible loss of
information in the more general cases and point out future directions of our
work.
