Penrose suggested in 1969 a possibility of extracting the rotational energy of a black hole by considering an infalling particle with energy E(in) that disintegrates in the ergosphere into two fragments. One fragment, with negative energy, falls into the hole, the other fragment escapes with the energy E(out) > E(in). However, shortly afterwards it was shown by Wald, Kovetz & Piran and others that in order that E(out) > E(in) the disintegration process must convert most of the rest mass energy of the infalling particle to kinetic energy.
Such a disintegration mechanism doesn't exist in nature, rendering the original Penrose process impossible.
Recently, a version of a "collisional" Penrose process has been considered by Banados, Silk & West (2009), who rediscovered the old result of Piran & Shaham (1977) that the center-of-mass energy of particles colliding at (nearly) the horizon of the maximally rotating Kerr black hole may be (formally) unbounded. Michał Bejger, Marek Abramowicz from the Copernicus Center and their collaborates show that although this statement is certainly correct, the center-of-mass energy is just an 'illusory' energy.
Indeed, one can have extremely energetic collisions very close to a black hole, but the results of such collisions plunge quickly into the black hole and a distant observer won't be able to know about them.
The maximal energy of an escaping particle is only a modest factor above the total initial energy of the colliding particles. This implies that one shouldn't expect any spectacular observational effects to arise from such collisions.
Animation shown: Michal Bursa
