The quantity coupled to the curvature is the stress-energy tensor. In local coordinates, this can be represented as a (1+n)x(1+n) matrix where n is the number o

The quantity coupled to the curvature is the stress-energy tensor. In local coordinates, this can be represented as a (1+n)x(1+n) matrix where n is the number of spatial dimensions, with components given by [mass density, mass flux = momentum density] [energy flux, stress] Photons are massless, but still carry momentum and energy, so they do gravitate.