The inverse square law in electrostatics is |F|=e2/((4πɛo)r2) for the force between an electron and a proton. The |F| can be understood in quantum theory as being due to the fact that the ‘particle’ of light (photon) is mass less. If photons had a mass mp, force would be modified to
Estimate the change in the ground state energy of a H-atom if mp were 10–6 times the mass of an electron.