High-energy collisions inside black holes and their counterpart in flat space-time

Two particles can collide inside a nonextremal black hole in such a way that the energy E_{c.m.} in their centre of mass frame becomes as large as one likes. We show that this effect can be understood with the help of a simple analogy with particle collisions in flat space-time. As the two-dimensional part of near-horizon geometry inside a black hole is described by the flat Milne metric, the results have a general character. Full classification of scenarios with unbound E_{c.m.} is suggested. Some scenarios of this kind require proximity of collision to the bifurcation point, but for some other ones this is not necessary.