  It has been suggested that maximally spinning black holes can serve as
particle accelerators, reaching arbitrarily high center-of-mass energies.
Despite several objections regarding the practical achievability of such high
energies, and demonstrations past and present that such large energies could
never reach a distant observer, interest in this problem has remained
substantial. We show that, unfortunately, a maximally spinning black hole can
never serve as a probe of high energy collisions, even in principle and despite
the correctness of the original diverging energy calculation. Black holes can
indeed facilitate dark matter annihilation, but the most energetic photons can
carry little more than the rest energy of the dark matter particles to a
distant observer, and those photons are actually generated relatively far from
the black hole where relativistic effects are negligible. Therefore, any strong
gravitational potential could probe dark matter equally well, and an appeal to
black holes for facilitating such collisions is unnecessary.
