title: Spinning black holes as particle accelerators.
abstract: It has recently been pointed out that particles falling freely from rest at infinity outside a Kerr black hole can in principle collide with an arbitrarily high center of mass energy in the limiting case of maximal black hole spin. Here we aim to elucidate the mechanism for this fascinating result, and to point out its practical limitations, which imply that ultraenergetic collisions cannot occur near black holes in nature.
authors.affiliation: Center for Fundamental Physics, University of Maryland, College Park, Maryland 20742-4111, USA.
authors.forename: Ted
authors.initials: T
authors.lastname: Jacobson
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Sotiriou