The free Maxwell equations imply that 
The precedent equation implies that the quantity  is constant. This time-independent quantity is one of the ten zilches discovered by Lipkin. Nowadays, the quantity  is widely known as optical chirality (up to a factor of 1/2). 
The quantity  is the spatial density of optical chirality, while   is the optical chirality flux. Generalizing the aforementioned differential conservation law for , Lipkin found other nine conservation laws, all unrelated to the stress–energy tensor. He collectively named these ten conserved quantities the zilch (nowadays, they are also called the zilches) because of the apparent lack of physical significance.