I am following Vaidya metric and how it is related to pure radiation from Wikipedia.(adsbygoogle = window.adsbygoogle || []).push({});

But when it reaches the line where stress-energy tensor is equated to product of two four-vectors, I cannot follow from where they are assumed to be null vectors, and why if the stress-energy tensor is given in terms of null vectors, it must be related to the energy of massless particles, or alternatively to particles with relativistic velocities, both of which are definitions of radiation.

What should be the components of stress-energy tensor in a given set of coordinates to say that it is related to pure radiation?

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# Understanding Vaidya metric and pure radiation stress-energy

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