A Photon-Mass Bound from Residual Dispersion in Fast Radio Bursts

A photon with a nonzero rest mass travels slower at lower frequencies, producing a frequency-dependent delay with the same inverse-frequency-squared shape as plasma dispersion but a different dependence on distance and on the plasma column. Using FRB 121102's line of sight — comoving distance 0.82 Gpc as computed here — and the 1.0 to 1.5 GHz band, a residual timing uncertainty of one millisecond after plasma dedispersion bounds the photon mass at 8.56e-16 eV per c-squared; burst structure sharp to a tenth of a millisecond tightens the figure to 2.71e-16 eV per c-squared. The degeneracy with plasma dispersion is the central difficulty, and this paper states exactly how it is broken in principle and how much of it survives in practice. The framework examined here assumes the standard massless photon; the bound is consistent with that assumption and quantifies how much mass the radio data tolerate. Computed from published data; not a new measurement.