The Pros and Cons of Time Travel

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? Additionally, Stephen Hawking once suggested that the absence of tourists from the future constitutes a strong argument against the existence of time travel. However, Einstein's theory of general relativity does suggest scientific grounds for thinking backwards time travel could be possible in certain unusual scenarios, although arguments from semi-classical gravity suggest that when quantum effects are incorporated into general relativity, these loopholes may be closed. These semi-classical arguments led Stephen 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.¹