Time Travel

Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime, or specific types of motion in space, may allow time travel into the past and future if these geometries or motions are possible.[3] Concepts that aid such understanding include the closed timelike curve.

Although the possibility of traveling to the future by moving at relativistic velocities is taken for granted by physicists, many in the scientific community believe that backwards time travel is highly unlikely. Any theory which would allow time travel would require that issues of causality be resolved. What if one were to go back in time and kill one's own grandfather (see grandfather paradox)? Additionally, Stephen Hawking once suggested that the absence of tourists from the future constitutes an argument against the existence of time travel--a variant of the Fermi paradox, with time travelers instead of alien visitors (but Hawking noted elsewhere that time travel may only be possible in a region of spacetime that is warped in the right way, and that if we can't create such a region until the future, then time travelers would not be able to travel back before that date, so 'This picture would explain why we haven't been over run by tourists from the future.'[4]) However, the theory of general relativity does suggest scientific grounds for thinking backwards time travel could be possible 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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