  We propose that observables in quantum theory are properly understood as
representatives of symmetry-invariant quantities relating one system to
another, the latter to be called a reference system. We provide a rigorous
mathematical language to introduce and study quantum reference systems, showing
that the orthodox "absolute" quantities are good representatives of observable
relative quantities if the reference state is suitably localised. We use this
relational formalism to critique the literature on the relationship between
reference frames and superselection rules, settling a long-standing debate on
the subject.

Quantum Physics