Towards Quantum Field Theory in Curved Spacetime for an Arbitrary Observer

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.