▁energy ▁condition ▁must ▁be ▁viol ated ▁on ▁the ▁C auch y ▁horizon ." ▁This ▁theorem ▁does ▁not ▁rule ▁out ▁the ▁possibility ▁of ▁time ▁travel ▁by ▁means ▁of ▁time ▁machines ▁with ▁the ▁non - comp actly ▁generated ▁C auch y ▁hor iz ons ▁( such ▁as ▁the ▁Deutsch - Pol itzer ▁time ▁machine ) ▁or ▁in ▁regions ▁which ▁contain ▁ex otic ▁matter , ▁which ▁would ▁be ▁used ▁for ▁travers able ▁w orm hol es ▁or ▁the ▁Alc ub ierre ▁drive . ▁ ▁In ▁January ▁ 2 0 2 0 , ▁ast ro phys ic ist ▁Ron ald ▁Mal let ▁proposed ▁a ▁theoretical ▁way ▁of ▁building ▁a ▁time ▁machine , ▁al beit ▁with ▁limitations , ▁based ▁on ▁general ▁relativ ity . ▁ ▁Quant um ▁physics ▁ ▁No - communic ation ▁theorem ▁ ▁When ▁a ▁signal ▁is ▁sent ▁from ▁one ▁location ▁and ▁received ▁at ▁another ▁location , ▁then ▁as ▁long ▁as ▁the ▁signal ▁is ▁moving ▁at ▁the ▁speed ▁of ▁light ▁or ▁slower , ▁the ▁mathematics ▁of ▁simultane ity ▁in ▁the ▁theory ▁of ▁relativ ity ▁show ▁that ▁all ▁reference ▁frames ▁agree ▁that ▁the ▁transmission - event ▁happened ▁before ▁the ▁reception - event . ▁When ▁the ▁signal ▁travel s ▁faster ▁than ▁light , ▁it ▁is ▁received ▁before ▁it ▁is ▁sent , ▁in ▁all ▁reference ▁frames . ▁The ▁signal ▁could ▁be ▁said ▁to ▁have ▁moved ▁back ward ▁in ▁time . ▁This ▁hypoth et ical ▁scenario ▁is ▁sometimes ▁referred ▁to ▁as ▁a ▁t ach y onic ▁ant ite le phone . ▁ ▁Quant um - me chan ical ▁phen omena