= Light-by-light scattering
The elastic scattering of two <photons> into two <photons>. In vacuum <QED> there is no photon self-coupling at tree level; its leading <scattering amplitude> is the sum of six oriented charged-fermion <box diagrams>. The <superficial degree of divergence> is zero, but the photon <Ward identities> forbid the required local dimension-four $A^4$ <counterterm>. The complete <one-loop Feynman diagram> sum is consequently <ultraviolet finite>. This does not remove its finite <four-momentum> dependence or its physical thresholds.
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