Relativity Seminar

If two particles collide near a rotating black hole, their energy in the center of mass frame E_c.m. can become unbounded under certain conditions. This is the Banados-Silk-West (BSW) effect. We give a brief review and general physical explanation from the geometric and kinematic viewpoints. In doing so, the Killing energy E of debris at infinity remains, in general, restricted. If E is also unbounded, this is called the super-Penrose process. We elucidate when such a process is possible and give full classification of corresponding relativistic objects for rotating space-times. In particular, we show that it is possible for rotating wormholes. The key role in consideration is played by the Wald inequalities. We also discuss briefly the case of a pure electric super-Penrose process near black holes and singularities. It is valid even in the flat space-time.

Although Penrose process remains a textbook example of a mechanism to extract energy from black holes, it has several practical limitations. A major problem is that it requires a particle to split into two fragments with very high relative velocity. This issue can be circumvented either by considering particles influenced by electromagnetic interaction or by switching to particle collisions instead of decays. Regarding the collisional Penrose process, a lot of attention has been devoted to idealised, best-case scenarios of test particle collisions with arbitrarily high centre-of-mass collision energies in the vicinity of extremal black holes. Bañados, Silk and West (BSW) first described such a possibility for particles with fine-tuned angular momentum orbiting a maximally rotating black hole. However, even in this simplified setup, the energy that can be extracted from the black hole turned out to be subjected to unconditional upper bounds. On the other hand, no such bounds were found... more