Photon Polarisation Decoherence and Birefringence over Gamma-Ray Burst Distances: Computed Limits

This paper computes the polarisation phase retardation accumulated by gamma-ray photons over gigaparsec distances under a quadratic Lorentz-violating birefringence model. The retardation is 6.404e+21 radians at one gigaelectronvolt, 6.404e+25 radians at one hundred gigaelectronvolts, and 6.404e+31 radians at one hundred teraelectronvolts, for unit strength over one gigaparsec. Observed polarisation at the highest energies therefore restricts the strength parameter severely, while low-energy polarisation carries almost no information in the quadratic model. The density ceiling plays no role in propagation through near-vacuum; the constraint is purely kinematic. The study is simulated and computed from stated assumptions; it is not a measurement.