The individual electron angular momenta l commute with this Hamiltonian. The velocities are expressed as functions of their corresponding momenta by inverting their defining relation. J.J. Coupling, which refers to the total angular momenta of individual particles. This is as desired for equally spaced angular momenta. In Cartesian coordinates, the generalized momenta are precisely the physical linear momenta. Note that canoncial momenta are not gauge invariant, and is not physically measurable. This system gives rise to a number of conserved quantities&colon; the angular momenta. It can be used to perform time reversal on angular momenta. Voronoi tessellations of the reciprocal lattice of momenta are called Brillouin zones. The phase space has generalized coordinates called , and associated canonical momenta called . The momentum of a system of particles is the vector sum of their momenta. The corresponding momenta to "q" will be conserved. In practice generalized coordinates are more convenient to use and interpret than generalized momenta. There is only one conserved quantity (the energy), and no conserved momenta. Newtonian momenta, calculated as , fail to behave properly under Lorentzian transformation.