▁infinite ▁frames ▁of ▁reference , ▁each ▁of ▁them ▁having ▁a ▁different ▁set ▁of ▁simultane ous ▁events , ▁which ▁makes ▁it ▁impossible ▁to ▁distinguish ▁a ▁single ▁" real " ▁present , ▁and ▁hence ▁either ▁all ▁events ▁in ▁time ▁are ▁real — bl urr ing ▁the ▁difference ▁between ▁present ism ▁and ▁et ernal ism — or ▁each ▁frame ▁of ▁reference ▁exists ▁in ▁its ▁own ▁reality . ▁Options ▁for ▁present ism ▁in ▁special ▁relativ ity ▁appear ▁to ▁be ▁exha usted , ▁but ▁Gö del ▁and ▁others ▁suspect ▁present ism ▁may ▁be ▁valid ▁for ▁some ▁forms ▁of ▁general ▁relativ ity . ▁Gener ally , ▁the ▁idea ▁of ▁absolute ▁time ▁and ▁space ▁is ▁considered ▁in compatible ▁with ▁general ▁relativ ity ; ▁there ▁is ▁no ▁universal ▁truth ▁about ▁the ▁absolute ▁position ▁of ▁events ▁which ▁occur ▁at ▁different ▁times , ▁and ▁thus ▁no ▁way ▁to ▁determine ▁which ▁point ▁in ▁space ▁at ▁one ▁time ▁is ▁at ▁the ▁universal ▁" same ▁position " ▁at ▁another ▁time , ▁and ▁all ▁coordinate ▁systems ▁are ▁on ▁equal ▁foot ing ▁as ▁given ▁by ▁the ▁principle ▁of ▁dif fe omorphism ▁in var iance . ▁ ▁The ▁grand father ▁par ado x ▁▁ ▁A ▁common ▁ob jection ▁to ▁the ▁idea ▁of ▁travel ing ▁back ▁in ▁time ▁is ▁put ▁forth ▁in ▁the ▁grand father ▁par ado x ▁or ▁the ▁argument ▁of ▁auto - inf antic ide . ▁If ▁one ▁were ▁able ▁to ▁go ▁back ▁in ▁time , ▁incons isten cies ▁and ▁contradict ions ▁would ▁ens ue ▁if ▁the ▁time ▁travel er ▁were ▁to ▁change ▁anything ; ▁there ▁is ▁a