A '''tachyon''' is a hypothetical particle that travels only faster than the [[speed of light]]. Tachyons cause non-causalities <ref>Phys. Rev. D 1, 3224 - 3225 (1970)</ref>. | A '''tachyon''' is a hypothetical particle that travels only faster than the [[speed of light]]. Tachyons cause non-causalities <ref>Phys. Rev. D 1, 3224 - 3225 (1970)</ref>. |