▁by ▁an ▁observer ▁passing ▁through ▁the ▁w orm hole , ▁no ▁matter ▁how ▁the ▁two ▁ends ▁move ▁around . ▁This ▁means ▁that ▁an ▁observer ▁entering ▁the ▁" you ng er " ▁end ▁would ▁exit ▁the ▁" older " ▁end ▁at ▁a ▁time ▁when ▁it ▁was ▁the ▁same ▁age ▁as ▁the ▁" you ng er " ▁end , ▁effectively ▁going ▁back ▁in ▁time ▁as ▁seen ▁by ▁an ▁observer ▁from ▁the ▁outside . ▁One ▁significant ▁limitation ▁of ▁such ▁a ▁time ▁machine ▁is ▁that ▁it ▁is ▁only ▁possible ▁to ▁go ▁as ▁far ▁back ▁in ▁time ▁as ▁the ▁initial ▁creation ▁of ▁the ▁machine ; ▁in ▁ess ence , ▁it ▁is ▁more ▁of ▁a ▁path ▁through ▁time ▁than ▁it ▁is ▁a ▁device ▁that ▁itself ▁moves ▁through ▁time , ▁and ▁it ▁would ▁not ▁allow ▁the ▁technology ▁itself ▁to ▁be ▁moved ▁back ward ▁in ▁time . ▁ ▁According ▁to ▁current ▁theories ▁on ▁the ▁nature ▁of ▁w orm hol es , ▁construction ▁of ▁a ▁travers able ▁w orm hole ▁would ▁require ▁the ▁existence ▁of ▁a ▁subst ance ▁with ▁negative ▁energy , ▁often ▁referred ▁to ▁as ▁" ex otic ▁matter ". ▁More ▁techn ically , ▁the ▁w orm hole ▁spac etime ▁requires ▁a ▁distribution ▁of ▁energy ▁that ▁viol ates ▁various ▁energy ▁conditions , ▁such ▁as ▁the ▁null ▁energy ▁condition ▁along ▁with ▁the ▁weak , ▁strong , ▁and ▁dominant ▁energy ▁conditions . ▁However , ▁it ▁is ▁known ▁that ▁quantum ▁effects ▁can ▁lead ▁to ▁small ▁meas urable ▁viol ations ▁of ▁the ▁null ▁energy ▁condition , ▁and ▁many ▁phys ic ists ▁believe ▁that ▁the ▁required