One of my professors, Arjendu Pattanayak, used to defer hairy questions in class with “That’s a beer question—meaning, buy me a beer at the pub, and we’ll talk about it.”

In that spirit, I will buy anyone who can explain this as many beers as it takes:

In general relativity, spacetime curvature is proportional to the stress-energy tensor T. T’s components are basically four-momentum density and flux, which are characterized as “momentum” and “energy”. Photons count. So do other particles with with four-momentum. Does anything else contribute? Electrostatics? The vector potential? The strong nuclear force? What is it that makes something contribute to T?