1999/01/01 by Marco Bruni, Sebastiano Sonego · 2 citations
Physics and Astronomy · #BRST quantization #Gauge theory #Hamiltonian lattice gauge theory #Introduction to gauge theory #Mathematical descriptions of the electromagnetic field #Noncommutative and Quantum Gravity Theories #Observable #Quantum and Classical Electrodynamics #Relativity and Gravitational Theory #Scalar (mathematics) #Scalar field #Spacetime #astro-ph #gr-qc #hep-th
paper · pdf · doi:10.1088/0264-9381/16/7/101
published as Class.Quant.Grav.16:L29-L36,1999 · 8 pages, LaTeX, CQG style. Classical and Quantum Gravity, Letters to the Editor, in press
openalex publication_date 1999/01/01 · arxiv created 1999/06/04 · arxiv updated 2010/04/06 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
We discuss the issue of observables in general-relativistic perturbation theory, adopting the view that any observable in general relativity is represented by a scalar field on spacetime. In the context of perturbation theory, an observable is therefore a scalar field on the perturbed spacetime, and as such is gauge invariant in an exact sense (to all orders), as one would expect. However, perturbations are usually represented by fields on the background spacetime, and expanded at different orders into contributions that may or may not be gauge independent. We show that perturbations of scalar quantities are observable if they are first-order gauge invariant, even if they are gauge dependent at higher order. Gauge invariance to first order therefore plays an important conceptual role in the theory, for it selects the perturbations with direct physical meaning from those having only a mathematical status. The so-called `gauge problem', and the relationship between measured fluctuations and gauge-dependent perturbations that are computed in the theory are also clarified.