  The frequency-dependent conductivity is studied for the one-dimensional
Hubbard model, using a selection rule, the Bethe ansatz, and symmetries
associated with conservation laws. For densities where the system is metallic
the absorption spectrum has two contributions, a Drude peak at $\omega = 0$
separated by a pseudogap from a broad absorption band whose lower edge is
characterized by a non-classical critical exponent. Our findings shed new light
on the ``far infrared puzzle'' and other optical properties of metallic organic
chain compounds.
