Forward Elastic Scattering of Light on Light, γ+γtoγ+γ

The forward elastic scattering of light on light, {\em i.e.,} the reaction $\gamma+\gamma \to \gamma+\gamma$ in the forward direction, is analyzed utilizing real analytic amplitudes. We calculate $\rho_{\gamma \gamma}$, the ratio of the real to the imaginary portion of the forward scattering amplitude, by fitting the total $\gamma \gamma$ cross section data in the high energy region $5 GeV \le \sqrt s \le 130 $ GeV, assuming a cross section that rises asymptotically as $\ln^2 s$. We then compare $\rho_{\gamma\gamma}$ to $\rho_{nn}$, the ratio of the even portions of the $pp$ and $\pbar p$ forward scattering amplitudes, as well as to $\rho_{\gamma p}$, the $\rho$ value for Compton scattering. Within errors, we find that the three $\rho$-values in the c.m.s. energy region $5 GeV \le \sqrt s \le 130$ GeV are the same, as predicted by a factorization theorem of Block and Kadailov.