title: Comment on "Kerr black holes as particle accelerators to arbitrarily high energy".
authors.affiliation: California Institute of Technology, Pasadena, California 91109, USA.
authors.forename: Emanuele
authors.initials: E
authors.lastname: Berti
authors.forename: Vitor
authors.initials: V
authors.lastname: Cardoso
authors.forename: Leonardo
authors.initials: L
authors.lastname: Gualtieri
authors.forename: Frans
authors.initials: F
authors.lastname: Pretorius
authors.forename: Ulrich
authors.initials: U
authors.lastname: Sperhake