I'm looking at 'Lecture Notes on General Relativity, Sean M. Carroll, 1997' Link here:http://arxiv.org/pdf/gr-qc/9712019.pdf Page 221 (on the actual lecture notes not the pdf), where it generalizes that the energy-momentum tensor for radiation - massive particles with velocities tending to the speed of light and EM radiation- can be expressed in terms of the field strength. So it says that at such speeds , the particles become indistinguishable from the speed of light as far as the equation of state is concerned. My Question: How are the energy-momentum tensor and equation of state related? How does it follow from this fact that the energy-momentum tensor of the particles takes the same form as photons do? Thanks in advance.