  We give a detailed microscopic analysis of why holes are different from
electrons in condensed matter. Starting from a single atom with zero, one and
two electrons, we show that the spectral functions for electrons and for holes
are qualitatively different because of electron-electron interactions. The
quantitative importance of this difference increases as the charge of the
nucleus decreases. Extrapolating our atomic analysis to the solid, we discuss
the expected differences in the single particle spectral function and in
frequency dependent transport properties for solids with nearly empty and
nearly full electronic energy bands. We discuss the expected dependence of
these quantities on doping, and the physics of superconductivity that results.
We also discuss how these features of the atomic physics can be modeled by a
variety of model Hamiltonians.
