▁main ▁bodies , ▁and ▁the ▁prec ursor ▁tra ve led ▁at ▁c ▁in ▁a ▁vac u um . ▁According ▁to ▁Du , ▁this ▁implies ▁that ▁there ▁is ▁no ▁possibility ▁of ▁light ▁travel ing ▁faster ▁than ▁c ▁and , ▁thus , ▁no ▁possibility ▁of ▁viol ating ▁caus ality . ▁ ▁Abs ence ▁of ▁time ▁travel ers ▁from ▁the ▁future ▁▁ ▁The ▁absence ▁of ▁time ▁travel ers ▁from ▁the ▁future ▁is ▁a ▁variation ▁of ▁the ▁F erm i ▁par ado x . ▁As ▁the ▁absence ▁of ▁extr ater rest rial ▁visitors ▁does ▁not ▁prove ▁they ▁do ▁not ▁exist , ▁so ▁the ▁absence ▁of ▁time ▁travel ers ▁fails ▁to ▁prove ▁time ▁travel ▁is ▁phys ically ▁impossible ; ▁it ▁might ▁be ▁that ▁time ▁travel ▁is ▁phys ically ▁possible ▁but ▁is ▁never ▁developed ▁or ▁is ▁c aut iously ▁used . ▁Carl ▁S agan ▁once ▁suggested ▁the ▁possibility ▁that ▁time ▁travel ers ▁could ▁be ▁here ▁but ▁are ▁dis gu ising ▁their ▁existence ▁or ▁are ▁not ▁recognized ▁as ▁time ▁travel ers . ▁Some ▁versions ▁of ▁general ▁relativ ity ▁suggest ▁that ▁time ▁travel ▁might ▁only ▁be ▁possible ▁in ▁a ▁region ▁of ▁spac etime ▁that ▁is ▁war ped ▁a ▁certain ▁way , ▁and ▁hence ▁time ▁travel ers ▁would ▁not ▁be ▁able ▁to ▁travel ▁back ▁to ▁earlier ▁regions ▁in ▁spac etime , ▁before ▁this ▁region ▁existed . ▁Stephen ▁Haw king ▁stated ▁that ▁this ▁would ▁explain ▁why ▁the ▁world ▁has ▁not ▁already ▁been ▁over run ▁by ▁" t our ists ▁from ▁the ▁future ." ▁ ▁Several ▁experiments ▁have ▁been ▁carried ▁out ▁to ▁try ▁to