Gamma-Ray Burst Spectral Lags Are Not Propagation: An Inconsistency Test Across Redshift

Long gamma-ray bursts show spectral lags: their pulses peak later at low energies than at high energies, by tenths of a second. If those lags were vacuum dispersion accumulated in propagation, the dispersion scale implied by each burst would have to be the same for all of them. Computing the implied linear scale for three representative cases — lags of 0.02, 0.3 and 2.0 s at redshifts 0.5, 1 and 4, across the standard 25-50 and 100-300 keV bands — gives values spreading over a factor of 13.8, and the lag's observed dependence on burst luminosity and pulse structure, rather than on the cosmological path factor (which grows by 7.2 between redshift 0.5 and 4), contradicts the propagation pattern directly. The lags are source physics, as the pulse-width energy relation independently indicates. The framework examined here, which forbids vacuum dispersion, requires this conclusion; the test shows the data require it too, without assuming the framework. This is a negative result for propagation readings of spectral lags, stated as such. Computed from published lag phenomenology; not a new measurement.