Quantum break in high intensity gravitational wave interactions

The lowest order amplitudes for [${\rm graviton+graviton \rightarrow photon +photon}]$ lead to cross-sections of order $G^2$, where $G$ is the gravitational constant. These are too small to be of any interest. However in dense clouds of pure gravitons there are collective effects utilizing these same amplitudes that under the right circumstances can lead to copious production of photons.