On Twitter someone spotted this paper

twitter.com/xavnet2/status/1360733192131670019



Illustrations of the relativistic conservation law for the center of energy
American Journal of Physics 73, 953 (2005); doi.org/10.1119/1.1900101
Timothy H. Boyer


ABSTRACT
The relativistic conservation law involving the center of energy is reviewed and illustrated using simple examples from classical electromagnetic theory. It is emphasized that this conservation law is parallel to the conservation laws for energy, linear momentum, and angular momentum, because it arises from the generators of the Poincare group for electromagnetic theory; yet this relativistic law reflecting the continuous flow of energy is not mentioned in text books. The illustrations include situations where external forces are present and are absent. A parallel plate capacitor, a flattened slip-joint solenoid, and two interacting charges are treated.

I don't understand all, but it may be related qui EmDrive&al.
Comments and vulgarisation is welcome.