Past exam of the mathematics course of the University of Cambridge / 2016 / ib / Paper 2 / 18D / b / Solution

Contractions of Lorentz tensors have no free indices and are Lorentz scalars. The antisymmetry and the components in part (a) giveThus $\mathbf{E}\cdot \mathbf{B}$ and $|\mathbf{B}{|}^2-|\mathbf{E}{|}^2/c^2$ are the two electromagnetic field invariants under proper Lorentz transformations. The second contraction uses the orientation-dependent epsilon tensor: strictly, $\mathbf{E}\cdot \mathbf{B}$ is a pseudoscalar, changing sign under an improper transformation such as spatial parity. Under the boosts in this question it is invariant.