  The standard theory of metals, Fermi liquid theory, hinges on the key
assumption that although the electrons interact, the low-energy excitation
spectrum stands in a one-to-one correspondence with that of a non-interacting
system. In the normal state of the copper-oxide high-temperature
superconductors, drastic deviations from the Fermi liquid picture obtain,
highlighted by a pseudogap, broad spectral features and $T-$ linear
resistivity. This article focuses on the series of experiments on the
copper-oxide superconductors which reveal that the number of low-energy
addition states per electron per spin exceeds unity, in direct violation of the
key Fermi liquid tenet. These experiments point to new degrees of freedom, not
made out of the elemental excitations, as the key mechanism by which Fermi
liquid theory breaks down in the cuprates. A recent theoretical advance which
permits an explicit integration of the high energy scale in the standard model
for the cuprates reveals the source of the new dynamical degrees of freedom at
low energies, a charge 2e bosonic field which has nothing to do with pairing
but rather represents the mixing with the high energy scales. We demonstrate
explicitly that at half-filling, this new degree of freedom provides a
dynamical mechanism for the generation of the charge gap, antiferromagnetism in
the insulating phase and explains many of the anomalies in the normal state of
the cuprates.

Strongly Correlated Electrons