The electromagnetic stress–energy tensor in terms of the displacement is: T α α π π = F α α β β D π π β β − − 1 4 δ δ α α π π F μ μ ν ν D μ μ ν ν , where δ α is the Kronecker delta. When the upper index is lowered with η, it becomes symmetric and is part of the source of the gravitational field.