  A rather elusive helicity reversal occurs in a gedanken experiment in which a
massive left-handed Dirac neutrino, traveling at a velocity u < c, is overtaken
on a highway by a speeding vehicle (traveling at velocity v with u < v < c).
Namely, after passing the neutrino, looking back, one would see a right-handed
neutrino (which has never been observed in nature). The Lorentz-invariant mass
of the right-handed neutrino is still the same as before the passing. The
gedanken experiment thus implies the existence of right-handed, light
neutrinos, which are not completely sterile. Furthermore, overtaking a bunch of
massive right-handed Dirac neutrinos leads to gradual de-sterilization. We
discuss the helicity reversal and the concomitant sterilization and
de-sterilization mechanisms by way of an illustrative example calculation, with
a special emphasis on massive Dirac and Majorana neutrinos. We contrast the
formalism with a modified Dirac neutrino described by a Dirac equation with a
pseudoscalar mass term proportional to the fifth current.
