> logically speaking result of the phase shift will be the effect that look like slowdown for outside observer. If there was a phase shift but no slowdown, wou

> logically speaking result of the phase shift will be the effect that look like slowdown for outside observer. If there was a phase shift but no slowdown, wouldn't we expect individual photons to propagate through a 10km loop of fibre optic cable without any slowdown, which would be measurable?