Momentum
From QED
Momenta are quantities in physical systems. Momenta are conserved (in Lagrangian and Hamiltonian systems) wherever there exists a symmetry in the potential along a generalised coordinate, or, equivalently, an absence of force along that coordinate.
-- okay, trying to disentangle canonical and physical and generalised momentum. I've never entirely gotten this.
This page was recovered in October 2009 from the Plasmagicians page on Momentum dated 00:51, 30 March 2007.