Which conjecture did Stephen Hawking propose about laws preventing time travel?

Stephen Hawking proposed the chronology protection conjecture, which holds that the laws of physics prevent macroscopic time travel.

General relativity permits mathematical solutions containing closed timelike curves—paths through spacetime that return to an earlier event. Hawking argued that quantum effects and the back-reaction of matter and radiation would prevent a usable time machine from forming, preserving ordinary cause-and-effect relationships.

His 1992 paper, “Chronology Protection Conjecture,” presented the idea in a deliberately playful way by referring to a hypothetical “Chronology Protection Agency.” The proposal is not a proven law of nature: a complete theory of quantum gravity would be needed to settle the issue definitively.

It is also distinct from the cosmic censorship hypothesis. Cosmic censorship concerns whether singularities are hidden behind event horizons, whereas chronology protection concerns the prevention of closed timelike curves and time-travel paradoxes. The no-boundary proposal addresses the universe’s early condition, not time travel.