Initial state propagators

Understand

It is possible to define a general initial state for a quantum field by introducing a contribution to the action defined at an initial-time boundary.

• The propagator for this theory is composed of two parts, one associated with the free propagation of fields and another produced by the operators of this initial action.
• The derivation of this propagator is shown for the case of a translationally and rotationally invariant initial state.
• In addition to being able to treat more general states, these techniques can also be applied to effective field theories that start from an initial time.