  We study the quantization of the curved spacetime created by
ultrarelativistic particles at Planckian energies. We consider a minisuperspace
model based on the classical shock wave metric generated by these particles,
and for which the Wheeler - De Witt equation is solved exactly. The wave
function of the geometry is a Bessel function whose argument is the classical
action. This allows us to describe not only the semiclassical regime
$(S\to\infty),$ but also the strong quantum regime $(S\to0).$ We analyze the
interaction with a scalar field $\phi$ and apply the third quantization
formalism to it. The quantum gravity effects make the system to evolve from a
highly curved semiclassical geometry (a gravitational wave metric) into a
strongly quantum state represented by a weakly curved geometry (essentially
flat spacetime).
