Full Idea
The essence of general relativity relates 'curvature in space-time' on one side of the equation to the 'density of momentum and energy' on the other. ...In full, Einstein required ten equations of this type.
Gist of Idea
The general relativity equations relate curvature in space-time to density of energy-momentum
Source
Frank Close (Theories of Everything [2017], 5 'Gravity')
A Reaction
Momentum involves mass, and energy is equivalent to mass (e=mc^2).
Book Reference
Close,Frank: 'Theories of Everything' [Profile Books 2017], p.96