The term massive particle refers to particles which have real non-zero rest mass. According to special relativity, their velocity is always lower than the speed of light. The synonyms bradyon (from Greek: βραδύς, bradys, “slow”), tardyon or ittyon are sometimes used to contrast with luxon (which moves at light speed) and hypothetical tachyon (which moves faster than light).
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- ^ Bilaniuk, O.-M.P.; Sudarshan, E.C.G. (1969). "Particles beyond the Light Barrier". Physics Today 22 (5): 43–51. Bibcode:1969PhT....22e..43B. doi:10.1063/1.3035574.