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According to the theory of relativity, particles having nonzero rest mass can approach, but not reach, the speed of light, as their mass would become infinite at that speed. On the other hand, particles with zero rest mass, like photons, always travel at the speed of light. Theorists have argued that nothing in principle prohibits the existence of a third class of particles, named tachyons, whose velocity always exceeds that of light. What effect would a loss of energy have on a tachyon’s speed? Discuss
Source: The Free Dictionary