  We study the question of conditions for the existence of negative-energy
states of particles in the absence of external fields in inertial and
noninertial frames of reference. We show that in the nonrelativistic case in
noninertial reference frames, there always exist domains where the energy of
particles is negative. We also show that in the relativistic case, the
existence of negative-energy states of point particles does not lead to
violations of the energy dominance condition. We consider conditions for the
appearance of negative and zero energies of particles in the Milne universe and
Rindler space-time.
