The
chronology protection conjecture is a
conjecture by
physicist Stephen Hawking that the
laws of physics are such as to prevent
time travel on all but submicroscopic scales. The permissibility of time travel is represented mathematically by the existence of
closed timelike curves. The chronology protection conjecture should be distinguished from chronological censorship under which every closed timelike curve passes through an
event horizon, which might prevent an observer from detecting the causal violation.