Breakdown of classical geometry for gravitational plane waves

Pure gravitational plane waves are described by a midisuperspace model that is free of constraints and whose degrees of freedom can be interpreted as a collection of annihilation and creation like variables. The system is qua ntised by introducing a Fock representation. In particular, we construct re gularised operators to represent the metric. For all coherent states in the quantum theory, including the vacuum, it is then shown that the classical description of the geometry breaks down close to the region where null cone s are focused Ly the wave.