Stephen Hawking's proposal that the laws of physics actively prevent time travel to the past, that any would-be time machine self-destructs at the moment of switching on.
Documented
The conjecture and its mechanism
Published in Physical Review D in 1992, the conjecture: the laws of physics prevent closed timelike curves from forming in any finite region. Hawking's evidence was the chronology horizon calculation, showing that near a time machine's threshold, quantum vacuum fluctuations circulate through the loop repeatedly, amplifying until the energy density blows up and seals the machine. Relativity had already shown time travel is mathematically legal, Godel's rotating universe (1949), traversable wormholes (Thorne and Morris, 1988), so the conjecture is physics' attempt to legislate against what its own equations permit.
Assessment: Documented as a serious, unproven proposal. The blow-up calculation works for specific setups and fails for others; later work showed the divergence can sometimes be evaded. Hawking framed it with his tongue in cheek, noting the absence of tourists from the future as empirical support.
Stories
The universe hates a paradox
The folk retelling is simple and comforting: nature will not let you kill your grandfather, full stop. It turns the conjecture into a cosmic referee and has been absorbed into fiction wholesale, every time machine in cinema now comes with a built-in doomsday clause, destiny, the universe correcting the anomaly.
Assessment: Documented as cultural absorption. The folk version overstates the physics: the conjecture bans machines that build loops, not necessarily loops built into spacetime, and says nothing about destiny. But it captured the field's mood perfectly, embarrassment at the loophole, faith that it will close.
Interesting claims
Or the loops were always there
The interesting claim is the conjecture's shadow: if chronology protection fails anywhere, causality was conventional, not fundamental. Novikov's self-consistency principle already describes a world where loops exist but only self-consistent ones; Deutsch's quantum version lets inconsistent branches spill into other timelines. On these readings Hawking's censor is not a wall but a border checkpoint that some physics has already walked around.
Assessment: Documented alternatives within mainstream physics, none confirmed. The conjecture's status today: unproven, unrefuted, and quietly load-bearing, because without it, general relativity's time machines stand legal and unexplained.
Sources and further reading
Hawking, Chronology Protection Conjecture, Physical Review D 46 (1992)
Godel, An Example of a New Type of Cosmological Solutions (1949), closed timelike curves
Morris and Thorne, Wormholes in Spacetime and Their Use for Interstellar Travel (1988)
Kim and Thorne, Do Vacuum Fluctuations Prevent the Creation of Closed Timelike Curves? (1991), a counter-case