2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/50073The existence of an electromagnetic field with parallel electric and magnetic field components in the presence of a gravitational field is considered. A non-parallel solution is shown to exist. Next, we analyse the possibility of finding stationary gravitational waves in nature. Finally, we construct a D=4 effective quantum gravity model. Tree-level unitarity is verified.6 pages, LaTexHigh Energy Physics - TheoryGeneral Relativity and Quantum Cosmologytext