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We study the question of conditions for the existence of negative-energystates of particles in the absence of external fields in inertial andnoninertial frames of reference. We show that in the nonrelativistic case innoninertial reference frames, there always exist domains where the energy ofparticles is negative. We also show that in the relativistic case, theexistence of negative-energy states of point particles does not lead toviolations of the energy dominance condition. We consider conditions for theappearance of negative and zero energies of particles in the Milne universe andRindler space-time.

Can the energy of a particle be negative in the absence of external fields?