  We introduce the concept of transport waves by showing that the linearized
Boltzmann transport equation admits excitations in the form of waves that have
well defined dispersion relations and decay times. Crucially, these waves do
not represent single-particle excitations, but are collective excitations of
the equilibrium distribution functions. We study in detail the case of thermal
transport, where relaxons are found in the long-wavelength limit, and second
sound is reinterpreted as the excitation of one or several temperature waves at
finite frequencies. Graphene is studied numerically, finding decay times of the
order of microseconds. The derivation, obtained by a spectral representation of
the Boltzmann equation, holds in principle for any crystal or semiclassical
transport theory and is particularly relevant when transport takes place in the
hydrodynamic regime.
