In physics, zilch (or zilches) is a set of ten conserved quantities of the source-free electromagnetic field, which were discovered by Daniel M. Lipkin in 1964. The name refers to the fact that the zilches are only conserved in regions free of electric charge, and therefore have limited physical significance. One of the conserved quantities (Lipkin's Z 0 ) has an intuitive physical interpretation and is also known as optical chirality.