Electromagnetic momentum density ($\mathbf{g}={ϵ}_0\mathbf{E}\times \mathbf{B}=\mathbf{S}/c^2$)

The electromagnetic momentum density in vacuum is $\mathbf{g}={ϵ}_0\mathbf{E}\times \mathbf{B}$. It equals the Poynting vector divided by $c^2$, expressing the relativistic relation between energy flux and momentum density.