Time travel to the past is theoretically possible in certain general
relativity spacetime geometries that permit traveling faster than
the speed of light, such as cosmic
strings, transversable wormholes, and Alcubierre
drive. The theory of general relativity does
suggest a scientific basis for the possibility of backward time travel in
certain unusual scenarios, although arguments from semiclassical gravity suggest that when quantum effects are incorporated into general
relativity, these loopholes may be closed.These semiclassical arguments led
Hawking to formulate the chronology protection conjecture,
suggesting that the fundamental laws of nature prevent time travel, but
physicists cannot come to a definite judgment on
the issue without a theory of quantum
gravity to join quantum
mechanics and general relativity into a completely unified theory.
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