- #36
DrDu
Science Advisor
- 6,372
- 979
I know too few about GR, but this apparent usefullness of non-invariant quantities arises in several other fields. E.g. the spin is defined as the angular momentum in the center of energy reference frame of a particle. However, this does not mean that spin is not defined for other reference frames but only that it cannot be interpreted as an angular momentum in these frames. Another example are the London equations of a superconductor, which contain explicitly the electromagnetic vector potential, however, only if formulated in Coulomb gauge. A chain of more and more trivial examples can be developed.