▁of ▁the ▁phot ons ▁had ▁moved ▁more ▁slowly ▁and ▁thus ▁contradict ing ▁the ▁theory ▁of ▁general ▁relativ ity ' s ▁notion ▁of ▁the ▁speed ▁of ▁light ▁being ▁constant , ▁a ▁dis cre p ancy ▁which ▁could ▁be ▁explained ▁by ▁the ▁irregular ity ▁of ▁quantum ▁fo am . ▁More ▁recent ▁experiments ▁were , ▁however , ▁unable ▁to ▁confirm ▁the ▁supposed ▁variation ▁on ▁the ▁speed ▁of ▁light ▁due ▁to ▁gra in iness ▁of ▁space . ▁ ▁Other ▁experiments ▁involving ▁the ▁polar ization ▁of ▁light ▁from ▁distant ▁g amma ▁ray ▁burst s ▁have ▁also ▁produced ▁contradict ory ▁results . ▁More ▁Earth - based ▁experiments ▁are ▁on going ▁or ▁proposed . ▁ ▁Con stra ints ▁and ▁limits ▁ ▁The ▁large ▁fl uct u ations ▁characteristic ▁of ▁a ▁spac etime ▁fo am ▁would ▁be ▁expected ▁to ▁occur ▁on ▁a ▁length ▁scale ▁on ▁the ▁order ▁of ▁the ▁Plan ck ▁length . ▁A ▁fo amy ▁spac etime ▁would ▁have ▁limits ▁on ▁the ▁accuracy ▁with ▁which ▁distances ▁can ▁be ▁measured ▁because ▁the ▁size ▁of ▁the ▁many ▁quantum ▁b ub bles ▁through ▁which ▁light ▁travel s ▁will ▁fl uct uate . ▁Depending ▁on ▁the ▁spac etime ▁model ▁used , ▁the ▁spac etime ▁un cert aint ies ▁accum ulate ▁at ▁different ▁rates ▁as ▁light ▁travel s ▁through ▁the ▁vast ▁distances . ▁ ▁X - ray ▁and ▁g amma - ray ▁observations ▁of ▁qu as ars ▁used ▁data ▁from ▁NASA ’ s ▁Ch andra ▁X - ray ▁Observ atory , ▁the ▁F erm i ▁G amma - ray ▁Space ▁T eles cope ▁and ▁ground