Colloquium: Planckian dissipation in metals

The appearance of the Planckian time τ_P1=ℏ/k_BT is reviewed in both conventional and unconventional metals. A pedagogical discussion of the various different timescales (quasiparticle, transport, many-body) that characterize metals is given, with an emphasis on conditions under which these times are the same or different. The possibility of a Planckian bound on dissipation is discussed from both a quasiparticle and a many-body perspective. Planckian quasiparticles can arise naturally from a combination of inelastic scattering and mass renormalization. Many-body dynamics, on the other hand, is constrained by the basic timescales and length scales of local thermalization.