... the components don’t change. Meanwhile, the three-dimensionalLevi-Civita tensor ijkis defined just as the four-dimensional one, although with one fewerindex. We have replaced the charge density ... to be the speed of light; the important thing, however, is not that photonshappen to travel at that speed, but that there exists a c such that the spacetime intervalis invariant under changes ... and lower indices with each other, so don’tsuccumb to the temptation to think of the Kronecker delta δαβas symmetric. On the otherhand, the fact that lowering an index on δαβgives a symmetric...