  Electrons and holes in clean, charge-neutral graphene behave like a strongly
coupled relativistic liquid. The thermo-electric transport properties of the
interacting Dirac quasiparticles are rather special, being constrained by an
emergent Lorentz covariance at hydrodynamic frequency scales. At small carrier
density and high temperatures, graphene exhibits signatures of a quantum
critical system with an inelastic scattering rate set only by temperature, a
conductivity with a nearly universal value, solely due to electron-hole
friction, and a very low viscosity. In this regime one finds pronounced
deviations from standard Fermi liquid behavior. These results, obtained by
Boltzmann transport theory at weak electron-electron coupling, are fully
consistent with the predictions of relativistic hydrodynamics. Interestingly,
very analogous behavior is found in certain strongly coupled relativistic
liquids, which can be analyzed exactly via the AdS-CFT correspondence, and
which had helped identifying and establishing the peculiar properties of
graphene.
