Radar Time and a State-Space Based Approach To Quantum Field Theory In   Gravitational and Electromagnetic Backgrounds
  In a recent paper (hep-th/0103228) a new initial value formulation of fermionic QFT was presented that is applicable to an arbitrary observer in any electromagnetic background. This approach suggests a consistent particle interpretation at all times, with the concept of `radar time' used to generalise this interpretation to an arbitrarily moving observer. In the present paper we extend this formalism to allow for gravitational backgrounds. The observer-dependent particle interpretation generalises Gibbons' definition to non-stationary spacetimes. This allows any observer to be considered, providing a particle interpretation that depends {\it only} on the observer's motion and the background, not on any choice of coordinates or gauge, or on details of their particle detector. Consistency with known results is demonstrated for the cases of Rindler space and deSitter space. Radar time is also considered for an arbitrarily moving observer in an arbitrary 1+1 dimensional spacetime, and for a comoving observer in a 3+1 dimensional FRW universe with arbitrary scale factor $a(t)$. Finite volume measurements and their fluctuations are also discussed, allowing one to say with definable precision where and when the particles are observed. 
Carl E. Dolby and Stephen F. Gull