▁discuss ▁the ▁possibility ▁of ▁closed ▁tim eli ke ▁curves , ▁which ▁are ▁world ▁lines ▁that ▁form ▁closed ▁loops ▁in ▁spac etime , ▁allowing ▁objects ▁to ▁return ▁to ▁their ▁own ▁past . ▁There ▁are ▁known ▁to ▁be ▁solutions ▁to ▁the ▁equations ▁of ▁general ▁relativ ity ▁that ▁describe ▁spac et imes ▁which ▁contain ▁closed ▁tim eli ke ▁curves , ▁such ▁as ▁Gö del ▁spac etime , ▁but ▁the ▁physical ▁pla us ibility ▁of ▁these ▁solutions ▁is ▁uncertain . ▁ ▁Many ▁in ▁the ▁scientific ▁community ▁believe ▁that ▁back ward ▁time ▁travel ▁is ▁highly ▁unlikely . ▁Any ▁theory ▁that ▁would ▁allow ▁time ▁travel ▁would ▁introduce ▁potential ▁problems ▁of ▁caus ality . ▁The ▁classic ▁example ▁of ▁a ▁problem ▁involving ▁caus ality ▁is ▁the ▁" grand father ▁par ado x ": ▁what ▁if ▁one ▁were ▁to ▁go ▁back ▁in ▁time ▁and ▁kill ▁one ' s ▁own ▁grand father ▁before ▁one ' s ▁father ▁was ▁conce ived ? ▁Some ▁phys ic ists , ▁such ▁as ▁Nov ik ov ▁and ▁Deutsch , ▁suggested ▁that ▁these ▁sorts ▁of ▁temporal ▁par ado xes ▁can ▁be ▁avoided ▁through ▁the ▁Nov ik ov ▁self - cons ist ency ▁principle ▁or ▁to ▁a ▁variation ▁of ▁the ▁many - world s ▁interpretation ▁with ▁interact ing ▁world s . ▁ ▁General ▁relativ ity ▁ ▁Time ▁travel ▁to ▁the ▁past ▁is ▁theoret ically ▁possible ▁in ▁certain ▁general ▁relativ ity ▁spac etime ▁geomet ries ▁that ▁permit ▁travel ing ▁faster ▁than ▁the ▁speed ▁of ▁light , ▁such ▁as ▁cos mic ▁strings , ▁trans vers able ▁w orm hol es , ▁and