  Any charged particle moving faster than light through a medium emits
Cherenkov radiation. We show that charged particles moving faster than light
through the v a c u u m emit Cherenkov radiation. How can a particle move
faster than light? The w e a k speed of a charged particle can exceed the speed
of light. By definition, the weak velocity is <b|v|a>/<b|a> where v is the
velocity operator and |a> and |b> are, respectively, the states of a particle
before and after a velocity measurement. We discuss the consistency of weak
values and show that superluminal weak speed is consistent with relativistic
causality.
