Stress–energy tensor

Summary

The stress–energy tensor is a symmetric tensor field of rank two on spacetime. It describes the density and the flow of energy and momentum of matter and fields. For an observer it gives the energy density, the momentum density and the stresses, such as pressure and shear, that this observer measures. It is the source term of the Einstein field equations, so it tells spacetime how to curve. Local conservation of energy and momentum is the statement that its divergence vanishes.