  The time delay between the receptions of ultra-relativistic particles emitted
simultaneously is a useful observable for both fundamental physics and
cosmology. The expression of the delay when the particles travel through an
arbitrary spacetime has been derived recently by Fanizza et al., using a
particular coordinate system and self-consistent assumptions. The present
article shows that this formula enjoys a simple physical interpretation: the
relative velocity between two ultra-relativistic particles is constant. This
result reveals an interesting kinematical property of general relativity,
namely that the tidal forces experienced by ultra-relativistic particles in the
direction of their motion are much smaller than those experienced orthogonally
to their motion.
