A Fringe-Shift Measurement of the Photon Circulation Radius: A Test of the Helical Identity mcrγ = ħ

We propose a test of the postulate that a photon carrying kinetic mass m = E/c2 circulates about its propagation axis, implying mcrγ = ħ. Because this identity follows from E = mc2 and E = hν, it is empirical only if rγ is measured independently of λ. We show that circulation coupling to slit edges imprints a helicity-odd phase ramp that rigidly shifts the far field by Δx/X = Crγ/a. The shift scales as λ and 1/a, preserves all fringe spacings, and vanishes for linear polarization. Phase metrology at 10−3–10−4 rad tests order-unity C at a fractional precision of 0.27–2.7% on rγ per frame-equivalent, reaching 0.04–0.39% after 50-frame averaging; failure of the helicity, scaling, or classical/quantum criterion falsifies the postulate.