It has recently been pointed out that particles falling freely from rest at
infinity outside a Kerr black hole can in principle collide with 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 ultra-energetic
collisions cannot occur near black holes in nature.Comment: 3 pages; v2: references added, minor modifications to match version
published in PR
