A scalar product for computing fundamental quantities in matter and its application to the helicity and angular momentum stored in a Hopfion

We introduce a systematic way to obtain expressions for computing the amount of fundamental quantities such as angular momentum contained in static matter, given its charge and magnetization densities. The expressions are obtained from a scalar product whose form results from imposing invariance under the scale-Euclidian group of transformations. Such group is obtained by restricting the conformal group of invariance of the dynamic Maxwell fields to the static case. In an exemplary application, we compute the helicity and angular momentum squared stored in a Hopfion, and show that the Hopfion is an eigenstate of angular momentum along one axis with eigenvalue zero.