  Photoemission and optical experiments indicate that the transition to
superconductivity in cuprates is an 'undressing' transition . In photoemission
this is seen as a coherent quasiparticle peak emerging from an incoherent
background, in optics as violation of the Ferrell-Glover-Tinkham sum rule
indicating effective mass reduction of superconducting carriers. We propose
that this is a manifestation of the fundamental electron-hole asymmetry of
condensed matter described by the theory of hole superconductivity. The theory
asserts that electrons in nearly empty bands and holes in nearly full bands are
fundamentally different : the former yield high conductivity and normal metals,
the latter yield low normal state conductivity and high temperature
superconductivity. This is because the normal state transport of electrons is
coherent and that of holes is incoherent. We explain how this asymmetry arises
from the Coulomb interaction between electrons in atoms and the nature of
atomic orbitals, and propose a simple Hamiltonian to describe it. A $universal$
mechanism for superconductivity follows from this physics, whereby dressed hole
carriers undress by pairing, turning (partially) into electrons and becoming
more mobile in the superconducting state.
