Gravitational effects of intense electromagnetic fields

In the presence of an electromagnetic (EM) field the space-time metrics are affected by the field potential. The principle of superposition, which usually rules EM fields, is no longer valid for the biggest fields and Maxwell's equations, for the free field, generally have a non-zero right-hand side. For weak potentials the metrics are very close to Euclidean metrics i.e. the gravitational field is negligible. Very high EM potentials and fields are necessary to create a measurable gravitational field and even higher to fail the principle of superposition. In this last case, the energy-impulsion tensor of the EM field has to be modified and the linearization of Einstein's equations is no longer possible.