title: Acausality of massive gravity.
abstract: We show, by analyzing its characteristics, that the ghost-free, 5 degree of freedom, Wess-Zumino massive gravity model admits superluminal shock wave solutions and thus is acausal. Ironically, this pathology arises from the very constraint that removes the (sixth) Boulware-Deser ghost mode.
authors.affiliation: Lauritsen Laboratory, Caltech, Pasadena, California 91125, USA and Physics Department, Brandeis University, Waltham, Massachusetts 02454, USA.
authors.forename: S
authors.initials: S
authors.lastname: Deser
authors.affiliation: Department of Mathematics, University of California, Davis, California 95616, USA.
authors.forename: A
authors.initials: A
authors.lastname: Waldron