  The conventional theory of superconductivity says that charge carriers in a
metal that becomes superconducting can be either electrons or holes. I argue
that this is incorrect. In order to satisfy conservation of mechanical momentum
and of entropy of the universe in the superconductor to normal transition in
the presence of a magnetic field it is necessary that the normal state charge
carriers are holes. I will also review the empirical evidence in favor of the
hypothesis that all superconductors are hole superconductors, and discuss the
implications of this for the search for higher $T_c$ superconductors.
