On gauge freedom and subsystems in quantum electrodynamics

I analyse different decompositions of composite light-matter systems into constituent
subsystems. I show that given a single description of a composite atomfield
system, many different decompositions into “atom” and “field” subsystems
can be made. I show that these decompositions are generally physically
inequivalent, and that they can be understood as corresponding to different
choices of gauge. I discuss some of the implications this has for the ontology
of QED, and find experimental signatures that could be used to distinguish
between different theoretical subsystem decompositions.