  It is generally believed that superconducting materials are divided into two
classes: `conventional' and `unconventional'. Conventional superconductors (the
elements and thousands of compounds including $MgB_2$) are described by
conventional London-BCS-Eliashberg electron-phonon theory. There is no general
agreement as to what mechanism or mechanisms describe `unconventional'
superconductors such as the heavy fermions, organics, cuprate and pnictide
families. However all superconductors, whether `conventional' or
`unconventional', exhibit the Meissner effect. I argue that there is a single
mechanism of superconductivity for all materials, that explains the Meissner
effect and differs from the conventional mechanism in several fundamental
aspects: it says that superconductivity is driven by lowering of kinetic rather
than potential energy of the charge carriers, it requires conduction by holes
rather than electrons in the normal state, and it predicts a non-homogeneous
rigid charge distribution and an electric field in the interior of
superconductors. Furthermore I argue that neither the conventional mechanism
nor any of the other proposed unconventional mechanisms can explain the
Meissner effect. Superconductivity in materials is discussed in the light of
these concepts, some experimental predictions, connections to Dirac's theory,
and connections to the superfluidity of $^4He$.
