Question

Is Newton's third law covariant?

Answer 1

Until now, there is no experimental evidence showing that Newton's third law has ever been violated in classical physics. ... But, the covarianceprinciple is not required by Physics as already stated by Phipps 30 and Cornille 3, 36 in both special and general relativity.

Answer 2

Newton's third law doesn't hold in a scalar field theory either, if that makes you feel better. The point is that conservation of momentum is the basic.