2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/54659The existence of an electromagnectic field with parallel electric and magnetic components is readdressed in the presence of a gravitational field. A non-parallel solution is shown to exist. Next, we analyse the possibility of finding stationary gravitational waves in de nature. Finaly, We construct a D=4 effective quantum gravity model. Tree-level unitarity is verified.6 pages, LatexHigh Energy Physics - TheoryGeneral Relativity and Quantum Cosmologytext