2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/35203We argue that if the velocity of electromagnetic waves in vacuum, \cem, is different from the limiting velocity for massive particles, \cst, then, a proton moving with velocity $\cem < v < \cst$ would radiate Cherenkov radiation. Because we have seen protons with energy $\sim 10^{19}$ eV coming from a distance of order more than 1 Mpc, we can put a constraint on $Δ:= (\cst - \cem)/\cst$: $Δ< 10^{-27}$.3 pagesGeneral Relativity and Quantum CosmologyAstrophysicsHigh Energy Physics - PhenomenologyHigh Energy Physics - Theorytext