Do Einstein's field equations explicitly show that energy alone can curve the metric of spacetime? True, energy is included in the stress-energy tensor, but is it assumed that energy in of itself curves spacetime? Or, is it possible that only energy "embedded" in mass contributes to curvature? The tensor also includes stress and pressure, but could we imagine stress or pressure without a mass to contain it? I think that would be impossible. Could we assert then that stress or pressure alone curves spacetime? I think it must be assumed that stress or pressure must manifest within existing mass in order for stress or pressure to contribute to curvature. So, is it possible that the same restriction applies to energy?