Acceleration of colliding shells around a black hole: Validity of the test particle approximation in the Banados-Silk-West process

Understand

Recently, Banados, Silk and West (BSW) showed that the total energy of two colliding test particles has no upper limit in their center of mass frame in the neighborhood of an extreme Kerr black hole, even if these particles were at rest at infinity in the infinite past.

• We call this mechanism the BSW mechanism or BSW process.
• The large energy of such particles would generate strong gravity, although this has not been taken into account in the BSW analysis.
• A similar mechanism is seen in the collision of two spherical test shells in the neighborhood of an extreme Reissner-Nordstr\"om black hole.