 Isotropic inverse-Compton drag

An isotropic photon bath with total radiation energy density $u_{\mathrm{ph}}$ exerts this drag on an electron in the Thomson scattering regime. Its stress-energy tensor has spatial diagonal entries $u_{\mathrm{ph}}/3$. A boost to the electron frame gives radiation energy flux $S_{\parallel }^{\prime }=-(4/3)c{\gamma }^2\beta u_{\mathrm{ph}}$; multiplying its radiation force ${\sigma }_T{S}_{\parallel }^{\prime }/c$ by the laboratory speed gives the electron's energy loss. This is net energy transferred, not all scattered photon energy. If only the magnetic wave-energy density is used, it is half of $u_{\mathrm{ph}}$ and the numerical coefficient doubles.