  It is generally believed that superconductivity can occur in materials
irrespective of whether the charge carriers in the material are electrons or
holes. Here we point out that Josephson tunneling would not occur between
superconductors with charge carriers of opposite sign. Consequently,
observation of Josephson tunneling between two superconductors implies that
their charge carriers have the same sign. We propose that this has profound
implications for the understanding of superconductivity and in particular is
consistent with the theory of hole superconductivity.
