2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/211901The spherically symmetric, static spacetime generated by a crossflow of non-interacting radiation streams, treated in the geometrical optics limit (null dust) is equivalent to an anisotropic fluid forming a radiation atmosphere of a star. This reference fluid provides a preferred / internal time, which is employed as a canonical coordinate. Among the advantages we encounter a new Hamiltonian constraint, which becomes linear in the momentum conjugate to the internal time (therefore yielding a functional Schrödinger equation after quantization), and a strongly commuting algebra of the new constraints.Section on boundary behavior and fall-off conditions of canonical variables added. New references, 1 new figure, 12 pages. Version accepted in Phys.Rev.DGeneral Relativity and Quantum CosmologyAstrophysicsHigh Energy Physics - TheoryQuantum Physicstext