Quantum field theory and observers

A generalization of the coadjoint orbit action describes the dynamics of an observer (or instrument). We consider how this fits in with the view of observables in field theory being correlations of readouts of instruments and show how one recovers the usual <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mi>S</a:mi> </a:mrow> </a:math> -matrix formulas. A simple resolution of the Fermi paradox is also pointed out.