Conservation of momentum can be derived as a necessary result in any theory whose basic laws are translation invariant (that is, if the defining equations are u

Conservation of momentum can be derived as a necessary result in any theory whose basic laws are translation invariant (that is, if the defining equations are unchanged if you move everything in the universe 6.2 cubits to the left). This is true in classical physics, relativistic dynamics, quantum mechanics, quantum field theory, or even string theory). You can prove it in various ways depending on which formalism you're working in, but the most elegant proofs I know are based on Noether's Theorem (http://math.ucr.edu/home/baez/noether.html http://math.ucr.edu/home/baez/noether.html). It's beautifully general, and it applies equally strongly to classical or quantum systems.