  We explore the collision between two concentric spherical thin shells. The
inner shell is charged, whereas the outer one is either neutral or charged. In
the situation we consider, the charge of the inner shell is larger than its
gravitational mass, and the inside of it is empty and regular. Hence the domain
just outside it is described by the overcharged Reissner-Nordstrom geometry
whereas the inside of it is Minkowski. First, the inner shell starts to shrink
form infinity with finite kinetic energy, and then the outer shell starts to
shrink from infinity with vanishing kinetic energy. The inner shell bounces on
the potential wall and collides with the ingoing outer shell. The energy of
collision between these shells at "their center of mass frame" does not exceed
the total energy of the system. By contrast, by virtue of the very large gamma
factor of the relative velocity of the shells, the energy of collision between
two of the constituent particles of these shells at their center of mass frame
can be much larger than the Planck scale. This result suggests that the black
hole or naked singularity is not necessary for ultra-high energy collision of
particles.
