  On 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 a multitude
of equivalent inertial reference frames. There is a class of experiments, in
which an inner observer can distinguish between the state of free fall in a
gravitational field and the state of free space by detecting the effect of a
proper mass variation. 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.
  Abbreviation: SRT- the Special Relativity Theory, GRT- the General Relativity
Theory, EP - the Equivalence Principle, PRS - the Pound-Rebka-Snider
(experiment)
