2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/2143On the basis of the relativistic mass-energy concept we found that a proper mass of a test particle in a gravitational field depends on a potential energy, hence, a freely falling particle has a varying proper mass. Consequently, a multitude of freely falling reference frames cannot be regarded as the multitude of equivalent inertial reference frames. There is a class of experiments, which allow distinguishing between them. If so, a demonstration of a violation of the Equivalence Principle is possible. It is shown that a variant of the classical Pound-Rebka-Snider experiment on a photon frequency shift in a gravitational field if conducted in a freely falling laboratory would be such a test.31 pagesAstrophysicstext