▁particles ▁emer ging ▁in ▁different ▁world s . ▁ ▁Ex perimental ▁results ▁ ▁C ertain ▁experiments ▁carried ▁out ▁give ▁the ▁impression ▁of ▁revers ed ▁caus ality , ▁but ▁fail ▁to ▁show ▁it ▁under ▁closer ▁exam ination . ▁ ▁The ▁delayed ▁choice ▁quantum ▁er aser ▁experiment ▁performed ▁by ▁Mar lan ▁S cul ly ▁involves ▁pairs ▁of ▁ent ang led ▁phot ons ▁that ▁are ▁divided ▁into ▁" signal ▁phot ons " ▁and ▁" id ler ▁phot ons ", ▁with ▁the ▁signal ▁phot ons ▁emer ging ▁from ▁one ▁of ▁two ▁locations ▁and ▁their ▁position ▁later ▁measured ▁as ▁in ▁the ▁double - sl it ▁experiment . ▁Depending ▁on ▁how ▁the ▁id ler ▁phot on ▁is ▁measured , ▁the ▁experiment er ▁can ▁either ▁learn ▁which ▁of ▁the ▁two ▁locations ▁the ▁signal ▁phot on ▁emer ged ▁from ▁or ▁" er ase " ▁that ▁information . ▁Even ▁though ▁the ▁signal ▁phot ons ▁can ▁be ▁measured ▁before ▁the ▁choice ▁has ▁been ▁made ▁about ▁the ▁id ler ▁phot ons , ▁the ▁choice ▁seems ▁to ▁retro act ively ▁determine ▁whether ▁or ▁not ▁an ▁inter ference ▁pattern ▁is ▁observed ▁when ▁one ▁correl ates ▁measurements ▁of ▁id ler ▁phot ons ▁to ▁the ▁corresponding ▁signal ▁phot ons . ▁However , ▁since ▁inter ference ▁can ▁be ▁observed ▁only ▁after ▁the ▁id ler ▁phot ons ▁are ▁measured ▁and ▁they ▁are ▁correl ated ▁with ▁the ▁signal ▁phot ons , ▁there ▁is ▁no ▁way ▁for ▁experiment ers ▁to ▁tell ▁what ▁choice ▁will ▁be ▁made ▁in ▁advance ▁just ▁by ▁looking ▁at ▁the ▁signal ▁phot ons , ▁only ▁by ▁gather ing ▁classical