A singularity arising from the coherent generation of gravitational waves by electromagnetic waves

The gravitational field arising from a circularly moving electromagnetic wave packet is considered and it is shown that the linearized Einstein's equations, on a flat spacetime background, predict infinitely large values for the curvature invariants at any points within the electromagnetic wave packet, where the phase speed of the wave packet equals the speed of light in vacuo. This singularity stems from a coherence effect, which comes into play whenever a source moves at the same speed as that of the disturbances it generates. (author)