The stress–energy tensor is a tensor physical quantity. A vector quantity is a tensor quantity that contains two components. A scalar quantity is a tensor quantity that contains only one component. These are special cases of tensors, being the simplest kind. But tensors can have any number n components; the stress-energy tensor having 16 – which by virtue of a partial symmetry reduces to 10.
The stress-energy tensor describes the density and flux of energy and momentum. In Newtonian gravity, it is the mass density which is the source of the gravitational field. In General Relativity, it is this density and flux of energy and momentum; the accompanying shear stress and pressure being important resulting components of the phenomena.
The stress-energy tensor generalizes the stress tensor of Newtonian physics to an attribute of matter, radiation, and non-gravitational force fields.
