On the time delay between ultrarelativistic particles
Abstract
The time delay between the receptions of ultrarelativistic 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 selfconsistent assumptions. The present article shows that this formula enjoys a simple physical interpretation: the relative velocity between two ultrarelativistic particles is constant. This result reveals an interesting kinematical property of general relativity, namely that the tidal forces experienced by ultrarelativistic particles in the direction of their motion are much smaller than those experienced orthogonally to their motion.
 Publication:

Physics Letters B
 Pub Date:
 September 2016
 DOI:
 10.1016/j.physletb.2016.07.010
 arXiv:
 arXiv:1604.03543
 Bibcode:
 2016PhLB..760..350F
 Keywords:

 General relativity;
 Neutrinos;
 Cosmic rays;
 General Relativity and Quantum Cosmology;
 Astrophysics  Cosmology and Nongalactic Astrophysics;
 High Energy Physics  Theory
 EPrint:
 5 pages, 1 figure, v2 matches published version