High-temperature superconductivity in the copper-oxide ceramics remains an
unsolved problem because we do not know what the propagating degrees of freedom
are in the normal state. As a result, we do not know what are the weakly
interacting degrees of freedom which pair up to form the superconducting
condensate. That the electrons are not the propagating degrees of freedom in
the cuprates is seen most directly from experiments that show spectral weight
redistributions over all energy scales. Of course, the actual propagating
degrees of freedom minimize such spectral rearrangements. This review focuses
on the range of epxerimental consequences such UV-IR mixings have on the normal
state of the cuprates, such as the pseudogap, mid-infrared band, temperature
dependence of the Hall number, the superfluid density, and a recent theoretical
advance which permits the identification of the weakly interacting degrees of
freedom in a doped Mott insulator. Within this theory, we show how the wide
range of phenomena which typify the normal state of the cuprates arises
including T−linear resistivity.Comment: To appear as a Colloquium in the April issue of Rev. Mod. Phys
Updated version contains new references and a clarification concerning Fig.
8