  Elastic light-by-light scattering, $\gamma\gamma\to\gamma\gamma$, is open to
study at the Large Hadron Collider thanks to the large quasi-real photon fluxes
available in electromagnetic interactions of protons (p) and lead (Pb) ions.
The $\gamma\gamma\to\gamma\gamma$ cross sections for diphoton masses
$m_{\gamma\gamma} > 5$ GeV amount to 105 fb, 260 pb, and 370 nb in p-p, p-Pb,
and Pb-Pb collisions at nucleon-nucleon center-of-mass energies $\sqrt{s_{NN}}$
= 14 TeV, 8.8 TeV, and 5.5 TeV respectively. Such a measurement has no
substantial backgrounds in Pb-Pb collisions where one expects about 70 signal
events per run, after typical detector acceptance and reconstruction efficiency
selections.
