Colloquium: Planckian dissipation in metals

Relaxational processes for conduction electrons in metals come with characteristic temperature dependences; classic examples at low temperatures are the 1/T^2 law for the relaxation time in a Fermi liquid, and the Bloch 1/T^5 law for electron-phonon scattering. The strange metal phase of cuprate superconductors is governed instead by the Planckian time, which is given in terms of fundamental constants only. This Colloquium discusses the concept of such a fundamental timescale and the degree to which it provides a lower bound for relaxation times, and illustrates these ideas with examples from conventional and unconventional metals.