If the energy-momentum tensor is that of an electromagnetic field in free space, i.e. if the electromagnetic stress-energy tensor .
R^{\alpha \beta} - \tfrac{1}{2}R g^{\alpha \beta} + \Lambda g^{\alpha \beta} = \frac{8 \pi G}{c^4 \mu_0} \left( {F^\alpha}^\psi {F_\psi}^\beta + \tfrac{1}{4} g^{\alpha \beta} F_{\psi\tau} F^{\psi\tau}\right).