| Assuming that the engineering problems
could be overcome, the production of a time machine could open up a Pandora's
box of causal paradoxes. Consider, for example, the time traveler who visits the
past and murders his mother when she was a young girl. How do we make sense of
this? If the girl dies, she cannot become the time traveler's mother. But if the
time traveler was never born, he could not go back and murder his
mother. Paradoxes of this kind arise when the time traveler tries to change the past, which is obviously impossible. But that does not prevent someone from being a part of the past. Suppose the time traveler goes back and rescues a young girl from murder, and this girls grows up to become his mother. The causal loop is now self-consistent and no longer paradoxical. Causal consistency might impose restrictions on what a time traveler is able to do, but it does not rule out time travel per second. Even if time travel isn't strictly paradoxical, it is certainly {{U}}weird{{/U}}. Consider the time traveler who leaps ahead a year and reads about a new mathematical theorem in a future edition of Scientific American. He notes the details, returns to his own time and teaches the theorem to a student, who then writes it up for Scientific American. The article is, of course, the very one that the time traveler reads. The question then arises: Where did the information about the theorem come from? Not from the time traveler, because he read it, but not from the student either, who learned it from the time traveler. The information seemingly came into existence from nowhere, reasonlessly. The bizarre consequences of time travel have led some scientists to reject the notion outright. Stephen W. Hawking of the University of Cambridge has proposed a "{{U}}Chronology{{/U}} protection conjecture," which would outlaw causal loops. Because the theory of relativity is known to permit causal loops, chronology protection would require some other factors to intercede to prevent travel into the past. What might this factor be? One suggestion is that quantum processes will come to the rescue. The existence of a time machine would allow particles to loop into their own past. Calculations hint that the ensuring disturbance would become self-reinforcing, creating a runaway surge of energy that would wreck the wormhole. |