Comment on "Black holes are neither particle accelerators nor dark matter probes".

The Banados-Silk-West effect consists in the possibility to get infinite energy in the centre of mass frame of two particles colliding near the black hole horizon. Ac-at infinity of the outcome vanishes because of infinite redshift when the point of collision approaches the horizon. I show that this is not so. The effect discovered by Bañados, Silk and West (BSW) states that if two particles collide near the black hole horizon, the energy in their centre of mass frame E c.m. can grow unbound [1]. For astrophysical purposes, it is important to know what can be detected at infinity as the products of collision. Here, there are two kinds of relevant quantities: (i) fluxes from a vicinity of the horizon, (ii) masses m and energies E ∞ of particles. In both cases strong redshift is crucial. Its account in a recent work [5] lead to the conclusions that (i) fluxes vanish due to relativistic dilatation of time, (ii) E ∞ → 0. Conclusion (i) looks reasonable and is essential for estimates of expected fluxes. However, (ii) is incorrect.