Is several other threads some arguments depend on what circumstances angular momentum is conserved in relativity and that is what I would like to establish here. This Wikipedia article http://en.wikipedia.org/wiki/Relativistic_angular_momentum describes relativistic angular momentum as being coupled with a vector quantity called the "time varying moment of mass" in a similar manner to the way energy and linear momentum couple to form the invariant four momentum. What exactly is the "moment of mass" in layman's terms? Is the four angular momentum conserved under a Lorentz transformation? Is the angular momentum part (p) of the exterior product x^p defined as ##p=m*r*u*\gamma (u) = m*r^2\omega*\gamma (\omega r)## ? As I understand it the the quantity ##p=m*r*u*\gamma(u)## is conserved for a system in a given reference frame (eg reduced r causes a corresponding increase in u maintaining p constant) but is not invariant under a transformation. Also, under what circumstances is angular momentum conserved in General Relativity? What does Noether's thereom tell us. I also found this paper http://panda.unm.edu/Courses/Finley/P495/TermPapers/relangmom.pdf [Broken] which states . Again, what is the "mass moment" (in practical terms) or centroid?