▁ 1 9 3 6 ▁and ▁K orn el ▁Lan cz os ▁in ▁ 1 9 2 4 , ▁but ▁not ▁recognized ▁as ▁allowing ▁closed ▁tim eli ke ▁curves ▁until ▁an ▁analysis ▁by ▁Frank ▁Ti pler ▁in ▁ 1 9 7 4 . ▁If ▁a ▁cyl inder ▁is ▁infinitely ▁long ▁and ▁sp ins ▁fast ▁enough ▁about ▁its ▁long ▁axis , ▁then ▁a ▁spaces hip ▁flying ▁around ▁the ▁cyl inder ▁on ▁a ▁spir al ▁path ▁could ▁travel ▁back ▁in ▁time ▁( or ▁forward , ▁depending ▁on ▁the ▁direction ▁of ▁its ▁spir al ). ▁However , ▁the ▁density ▁and ▁speed ▁required ▁is ▁so ▁great ▁that ▁ordinary ▁matter ▁is ▁not ▁strong ▁enough ▁to ▁construct ▁it . ▁A ▁similar ▁device ▁might ▁be ▁built ▁from ▁a ▁cos mic ▁string , ▁but ▁none ▁are ▁known ▁to ▁exist , ▁and ▁it ▁does ▁not ▁seem ▁to ▁be ▁possible ▁to ▁create ▁a ▁new ▁cos mic ▁string . ▁Phys ic ist ▁Ron ald ▁M alle tt ▁is ▁attempting ▁to ▁re create ▁the ▁conditions ▁of ▁a ▁rot ating ▁black ▁hole ▁with ▁ring ▁las ers , ▁in ▁order ▁to ▁b end ▁spac etime ▁and ▁allow ▁for ▁time ▁travel . ▁ ▁A ▁more ▁fundamental ▁ob jection ▁to ▁time ▁travel ▁schemes ▁based ▁on ▁rot ating ▁cyl ind ers ▁or ▁cos mic ▁strings ▁has ▁been ▁put ▁forward ▁by ▁Stephen ▁Haw king , ▁who ▁proved ▁a ▁theorem ▁showing ▁that ▁according ▁to ▁general ▁relativ ity ▁it ▁is ▁impossible ▁to ▁build ▁a ▁time ▁machine ▁of ▁a ▁special ▁type ▁( a ▁" time ▁machine ▁with ▁the ▁compact ly ▁generated ▁C auch y