The electromagnetic stress–energy tensor can be interpreted as the flux density of the momentum four-vector, and is a contravariant symmetric tensor that is the contribution of the electromagnetic fields to the overall stress–energy tensor : T α α β β = ( ε ε 0 E 2 / 2 + B 2 / 2 μ μ 0 S x / c S y / c S z / c S x / c − − σ σ x x − − σ σ x y − − σ σ x z S y / c − − σ σ y x − − σ σ y y − − σ σ y z S z / c − − σ σ z x − − σ σ z y − − σ σ z z ) , where ε ε 0 is the electric permittivity of vacuum, μ 0 is the magnetic permeability of vacuum, the Poynting vector is S = 1 μ μ 0 E × × B and the Maxwell stress tensor is given by σ σ i j = ε ε 0 E i E j + 1 μ μ 0 B i B j − − ( 1 2 ε ε 0 E 2 + 1 2 μ μ 0 B 2 ) δ δ i j .