  We study linear scaling relations in electron-phonon superconductors. By
combining numerical and analytical techniques, we find linear Homes scaling
relations between the zero-temperature superfluid density and the normal-state
DC conductivity. This is due to Galilean invariance being broken, either via a
large impurity scattering rate or inelastic scattering of electrons and
Einstein phonons at large electron-phonon coupling. Our work thus shows that
Homes scaling is more universal than either cuprate or BCS-like physics, and is
instead a fundamental result in a wide class of superconductors.
