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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- Folman, R.; Recami, E. (1995). "On the Phenomenology of Tachyon Radiation". Foundations of Physics Letters. 8 (2): 127–134. arXiv:hep-th/9508166. Bibcode:1995FoPhL...8..127F. doi:10.1007/BF02187583.
- 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.
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