chronology protection conjecture

The chronology protection conjecture is a proposition put forward by Stephen Hawking in 1991 according to which quantum effects will always conspire effectively to prevent time travel where classical physics, including the general theory of relativity, might otherwise allow time travel to occur. The chronology protection conjecture (CPC) states that solutions to the general theory that permit closed time-like paths are physically meaningless. Hawking quipped that CPC makes "the universe safe for historians" and provides one possible answer to the question "Where are the hordes of tourists from the future?"