  The need for a time-shift invariant formulation of quantum theory arises from
fundamental symmetry principles as well as heuristic cosmological
considerations. Such a description then leaves open the question of how to
reconcile global invariance with the perception of change, locally. By
introducing relative time observables, we are able to make rigorous the
Page-Wootters conditional probability formalism to show how local Heisenberg
evolution is compatible with global invariance.
