Equivalent Theories Redefine Hamiltonian Observables to Exhibit Change in General Relativity

TL;DR

This paper redefines Hamiltonian observables in General Relativity using equivalent formulations, showing that observables must exhibit change and covariance, not invariance, thus addressing the problem of time and the nature of observables.

Contribution

It introduces a new definition of observables based on equivalent formulations, demonstrating that observables in General Relativity can and do change, challenging traditional invariance-based views.

Findings

Observables must have zero Poisson bracket with the gauge generator G.

In massive gravity, gauge equivalent observables exhibit Lie derivatives, not zero Poisson brackets.

Local fields like g_{ ext{μν}} are observables that change over time.

Abstract

Change and local spatial variation are missing in canonical General Relativity's observables as usually defined, part of the problem of time. Definitions can be tested using equivalent formulations, non-gauge and gauge, because they must have equivalent observables and everything is observable in the non-gauge formulation. Taking an observable from the non-gauge formulation and finding the equivalent in the gauge formulation, one requires that the equivalent be an observable, constraining definitions. For massive photons, the de Broglie-Proca non-gauge formulation observable A_{\mu} is equivalent to the Stueckelberg-Utiyama gauge formulation quantity A_{\mu}+\partial_{\mu} \phi. Thus observables must have 0 Poisson bracket not with each first-class constraint, but with the Rosenfeld-Anderson-Bergmann-Castellani gauge generator G, a tuned sum of first-class constraints, in accord with the Pons-Salisbury-Sundermeyer definition of observables. The definition for external gauge symmetries can be tested using massive gravity, where one can install gauge freedom by parametrization with clock fields X^A. The non-gauge observable g^{\mu\nu} has the gauge equivalent X^A,_{\mu} g^{\mu\nu} X^B,_{\nu}. The Poisson bracket of X^A,_{\mu} g^{\mu\nu} X^B,_{\nu} with G turns out to be not 0 but a Lie derivative. This non-zero Poisson bracket refines and systematizes Kuchar's proposal to relax the 0 Poisson bracket condition with the Hamiltonian constraint. Thus observables need covariance, not invariance, in relation to external gauge symmetries. The Lagrangian and Hamiltonian for massive gravity are those of General Relativity + Lambda + 4 scalars, so the same definition of observables applies to General Relativity. Local fields such as g_{\mu\nu} are observables. Thus observables change. Requiring equivalent observables for equivalent theories also recovers Hamiltonian-Lagrangian equivalence.