Mathematical Definition
The Einstein tensor of a null dust must have the form where is a null vector field. This definition makes sense in the absence of any physical interpretation, but if we place a stress-energy tensor on our spacetime which happens to have the form then Einstein's field equation is trivially satisfied, and in addition, such a stress-energy tensor has a clear physical interpretation in terms of massless radiation. The vector field specifies the direction in which the radiation is moving; the scalar multiplier specifies its intensity.
Read more about this topic: Null Dust Solution
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