▁that ▁any ▁actions ▁taken ▁by ▁a ▁time ▁travel er ▁or ▁by ▁an ▁object ▁that ▁travel s ▁back ▁in ▁time ▁were ▁part ▁of ▁history ▁all ▁along , ▁and ▁therefore ▁it ▁is ▁impossible ▁for ▁the ▁time ▁travel er ▁to ▁" change " ▁history ▁in ▁any ▁way . ▁The ▁time ▁travel er ' s ▁actions ▁may ▁be ▁the ▁cause ▁of ▁events ▁in ▁their ▁own ▁past ▁though , ▁which ▁leads ▁to ▁the ▁potential ▁for ▁circular ▁caus ation , ▁sometimes ▁called ▁a ▁pred est ination ▁par ado x , ▁ont ological ▁par ado x , ▁or ▁bootstrap ▁par ado x . ▁The ▁term ▁bootstrap ▁par ado x ▁was ▁popular ized ▁by ▁Robert ▁A . ▁Hein lein ' s ▁story ▁" By ▁His ▁Boot stra ps ". ▁The ▁Nov ik ov ▁self - cons ist ency ▁principle ▁propos es ▁that ▁the ▁local ▁laws ▁of ▁physics ▁in ▁a ▁region ▁of ▁spac etime ▁containing ▁time ▁travel ers ▁cannot ▁be ▁any ▁different ▁from ▁the ▁local ▁laws ▁of ▁physics ▁in ▁any ▁other ▁region ▁of ▁spac etime . ▁ ▁The ▁phil os opher ▁K elle y ▁L . ▁Ross ▁arg ues ▁in ▁" Time ▁Tra vel ▁Par ado xes " ▁that ▁in ▁a ▁scenario ▁involving ▁a ▁physical ▁object ▁whose ▁world - line ▁or ▁history ▁forms ▁a ▁closed ▁loop ▁in ▁time ▁there ▁can ▁be ▁a ▁viol ation ▁of ▁the ▁second ▁law ▁of ▁therm od ynam ics . ▁Ross ▁uses ▁" Some where ▁in ▁Time " ▁as ▁an ▁example ▁of ▁such ▁an ▁ont ological ▁par ado x , ▁where ▁a ▁watch ▁is ▁given ▁to ▁a ▁person