Fleury, Pierre2023-06-052023-06-052016Fleury, P. (2016). On the time delay between ultra-relativistic particles. Physics Letters B, 760, 350-353. https://doi.org/10.1016/j.physletb.2016.07.0101873-2445https://doi.org/10.1016/j.physletb.2016.07.010http://hdl.handle.net/10566/9017The 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.enPhysicsCosmologyNeutrinosAstrophysicsMathematicsArticle