  Einstein's theory of special relativity(SR) and the principle of causality
imply that the speed of any moving object can not exceed that of light in a
vacuum($c$).However,there were many attempts in literature discussing the
particle moving with speed $u>c$(called as superluminal particle or
tachyon)either in the scheme of SR or beyond it.These theories all encountered
a series of insurmountable difficulties which will be named ``superluminal
paradox''in this paper.We will analyze it in some detail and then prove that
the paradox disappears unambiguously in quantum theory,which is compatible with
SR.Most likely,the superluminal particle in real world is just a kind of known
particle,the neutrino.
