Principle of covariance

In physics, the principle of covariance emphasizes formulating physical laws using only those quantities whose measurements can be unambiguously correlated by observers in different frames of reference.

Mathematically, the physical quantities must transform covariantly, that is, under a certain representation of the group of coordinate transformations between admissible frames of reference of the physical theory. This group is referred to as the covariance group.

The principle of covariance does not require the physical laws to be invariant under the group of admissible transformations, although in most cases the equations are indeed invariant. However, in the theory of weak interactions, the equations are not invariant under reflections (but are, of course, still covariant).