  The critical dynamics of superconductors is studied using renormalization
group and duality arguments. We show that in extreme type II superconductors
the dynamic critical exponent is given exactly by $z=3/2$. This result does not
rely on the widely used models of critical dynamics. Instead, it is shown that
$z=3/2$ follows from the duality between the extreme type II superconductor and
a model with a critically fluctuating gauge field. Our result is in agreement
with Monte Carlo simulations.
