▁there ▁was ▁likely ▁as ▁yet ▁no ▁acceptable ▁proof ▁of ▁the ▁second ▁post ulate ▁of ▁special ▁relativ ity . ▁ ▁This ▁surprising ▁gap ▁in ▁the ▁experimental ▁record ▁was ▁quickly ▁closed ▁in ▁the ▁en su ing ▁years , ▁by ▁experiments ▁by ▁Fox , ▁and ▁by ▁Al v ager ▁et ▁al ., ▁which ▁used ▁g amma ▁ray s ▁s our ced ▁from ▁high ▁energy ▁mes ons . ▁ ▁The ▁high ▁energy ▁levels ▁of ▁the ▁measured ▁phot ons , ▁along ▁with ▁very ▁careful ▁account ing ▁for ▁ext inction ▁effects , ▁elimin ated ▁any ▁significant ▁doubt ▁from ▁their ▁results . ▁ ▁Many ▁other ▁tests ▁of ▁special ▁relativ ity ▁have ▁been ▁conducted , ▁testing ▁possible ▁viol ations ▁of ▁Lor ent z ▁in var iance ▁in ▁certain ▁variations ▁of ▁quantum ▁gravity . ▁However , ▁no ▁sign ▁of ▁an is ot ropy ▁of ▁the ▁speed ▁of ▁light ▁has ▁been ▁found ▁even ▁at ▁the ▁ 1 0 − 1 7 ▁level , ▁and ▁some ▁experiments ▁even ▁ruled ▁out ▁Lor ent z ▁viol ations ▁at ▁the ▁ 1 0 − 4 0 ▁level , ▁see ▁Modern ▁searches ▁for ▁Lor ent z ▁viol ation . ▁ ▁Prior ity ▁Some ▁claim ▁that ▁Po inc ar é ▁and ▁Lor ent z , ▁not ▁Ein stein , ▁are ▁the ▁true ▁found ers ▁of ▁special ▁relativ ity . ▁For ▁more ▁see ▁the ▁article ▁on ▁relativ ity ▁priority ▁dispute . ▁ ▁Crit ic isms ▁ ▁Some ▁critic ized ▁Special ▁Rel ativity ▁for ▁various ▁reasons , ▁such ▁as ▁lack ▁of ▁empir ical ▁evidence , ▁internal ▁incons isten cies , ▁re