Kinematic Censorship as a Constraint on Allowed Scenarios of High-Energy Particle Collisions

In the recent years, it was found that the energy E-c.m. in the center of mass frame of two colliding particles can be unbounded near black holes. If a collision occurs precisely on the horizon, E-c.m. is formally infinite. However, in any physically reasonable situation this is impossible. We collect different scenarios of this kind and show why in every act of collision E-c.m. is indeed finite (although it can be as large as one likes). The factors preventing an infinite energy are diverse: the necessity of infinite proper time, infinite tidal forces, potential barrier, etc. This prompts us to formulate a general principle according to which the limits in which E-c.m. -> infinity are never achieved. We call this the kinematic censorship (KC). Although by itself the validity of KC is quite natural, its application allows one to forbid scenarios of collisions predicting infinite E-c.m. without going into details. The KC is valid even in the test particle approximation, so an explanation of why E-c.m. cannot be infinite does not require references (common in the literature) to a nonlinear regime, back-reaction, etc. The KC remains valid not only for freely moving particles but also if particles are subject to a finite force. For an individual particle, we consider a light-like continuous limit of a timelike trajectory in which the effective mass turns to zero. We show that it cannot be accelerated to an infinite energy during a finite proper time under the action of such a force. As an example, we consider the dynamics of a scalar particle interacting with a background scalar field.