Gauge Structure from Relational Dynamics

Abstract

We observe that the ingredients a relational construction already requires contain a gauge structure, and that its group is the one the frame cannot see. LetM= Gbig/H be a homogeneous space. Three independently established facts close into a statement when placed together. An operational quantum reference frame is a covariant POVM on a homogeneous space Σ ∼= G/Hx, and the relativization map expressing a system observable relative to it is defined only on the Hx-invariant part of the system algebra. The canonical connection of a reductive homogeneous space is h-valued: the projection of the Maurer–Cartan form onto the isotropy algebra. And the isotropy directions carry zero Fisher–Bures weight, which, that metric being the maximal monotone one, is the first statement in the vocabulary of the geometry. Four consequences. Gauge invariance is not imposed on the relational description; it is the condition under which an observable can be expressed relative to a frame at all. The Cartan decomposition splits what the frame cannot resolve from what it measures. The structure is non-abelian whenever the isotropy is, and for Sp(2n,R)/U(n) the isotropy is U(n). And the size of that isotropy and the multiplicities of its content are fixed by the field content, NB = 2 dimG+nH, a gauge sector carrying multiplicity two because the adjoint is real and a charged scalar one because the charge is complex. We then measure what the gauge algebra’s commutant inside the isotropy retains of it: the double centraliser is the full type-preserving algebra L λ u(dλ), of dimension 82 against the gauge algebra’s 12. The commutant does not remember the gauge group; it remembers the isotypic decomposition.No gauge group is derived here, and nothing is identified with anything physical.