Judging by the only thing man has been able to travel at the speed of light. It would cast no shadow, because ( as already stated ) it has to be converted to a light frequency ( or pigged backed on light ) to travel that fast. When 2 light source cross paths, they do not create a shadows. An object traveling at the speed of light would not cast a shadow when crossing another light scoure. But in the wonderful world of CG, there are no laws of physics or nature to break. So anything is possible. The thing is how realistic can it be made to look.

Stephen Ray