Maxwell's equations predict that the speed of EM waves ($c$) in a vacuum is determined by the fundamental constants of electricity (permittivity ${ϵ}_0$) and magnetism (permeability ${\mu }_0$): $c=\frac{1}{\sqrt{{ϵ}_0{\mu }_0}}$ Substituting ${ϵ}_0\approx 8.854\times 10^{-12}\text{ F/m}$ and ${\mu }_0=4\pi \times 10^{-7}\text{ H/m}$ gives $c\approx 3\times 10^8\text{ m/s}$.