, ▁such ▁as ▁rotation ▁and ▁lack ▁of ▁Hub ble ▁expansion . ▁Whether ▁general ▁relativ ity ▁forb ids ▁closed ▁time - like ▁curves ▁for ▁all ▁real istic ▁conditions ▁is ▁still ▁being ▁research ed . ▁ ▁W orm hol es ▁▁ ▁W orm hol es ▁are ▁a ▁hypoth et ical ▁war ped ▁spac etime ▁permitted ▁by ▁the ▁Ein stein ▁field ▁equations ▁of ▁general ▁relativ ity . ▁A ▁proposed ▁time - tra vel ▁machine ▁using ▁a ▁travers able ▁w orm hole ▁would ▁hypoth et ically ▁work ▁in ▁the ▁following ▁way : ▁One ▁end ▁of ▁the ▁w orm hole ▁is ▁acceler ated ▁to ▁some ▁significant ▁fraction ▁of ▁the ▁speed ▁of ▁light , ▁perhaps ▁with ▁some ▁advanced ▁prop ulsion ▁system , ▁and ▁then ▁brought ▁back ▁to ▁the ▁point ▁of ▁origin . ▁Alternatively , ▁another ▁way ▁is ▁to ▁take ▁one ▁entrance ▁of ▁the ▁w orm hole ▁and ▁move ▁it ▁to ▁within ▁the ▁gravit ational ▁field ▁of ▁an ▁object ▁that ▁has ▁higher ▁gravity ▁than ▁the ▁other ▁entrance , ▁and ▁then ▁return ▁it ▁to ▁a ▁position ▁near ▁the ▁other ▁entrance . ▁For ▁both ▁these ▁methods , ▁time ▁d ilation ▁causes ▁the ▁end ▁of ▁the ▁w orm hole ▁that ▁has ▁been ▁moved ▁to ▁have ▁aged ▁less , ▁or ▁become ▁" you ng er ", ▁than ▁the ▁station ary ▁end ▁as ▁seen ▁by ▁an ▁external ▁observer ; ▁however , ▁time ▁connect s ▁differently ▁through ▁the ▁w orm hole ▁than ▁outside ▁it , ▁so ▁that ▁synchron ized ▁clock s ▁at ▁either ▁end ▁of ▁the ▁w orm hole ▁will ▁always ▁remain ▁synchron ized ▁as ▁seen