If the energy–momentum tensor T μν is that of an electromagnetic field in free space, i.e. if the electromagnetic stress–energy tensor T α α β β = − − 1 μ μ 0 ( F α α ψ ψ F ψ ψ β β + 1 4 g α α β β F ψ ψ τ τ F ψ ψ τ τ ) is used, then the Einstein field equations are called the Einstein–Maxwell equations (with cosmological constant Λ, taken to be zero in conventional relativity theory): G α α β β + Λ Λ g α α β β = κ κ μ μ 0 ( F α α ψ ψ F ψ ψ β β + 1 4 g α α β β F ψ ψ τ τ F ψ ψ τ τ ) .