Chronology Protection Conjecture
There are those who believe that because of paradoxes time travel is impossible. Dr. Stephen Hawking of Cambridge found himself amongst those people. In fact, he formulated what he called the Chronology Protection Conjecture. It simply states that the universe will protect itself from time paradox by not allowing time machines to be made. The way the universe would accomplish this is through a variety of means. For instance, the universe might stop Kip Thorne and his wife from making a time machine out of a wormhole by destroying the wormhole with electro-magnetic vacuum fluctuations that feed back on themselves through the wormhole. Makers of cosmic string time machines would be cut-off from their time machines before they could use them by singularities or horizons. It seems to me that this conjecture is rather contrived. In the known physical cases of Closed Timelike Curves there are problems with horizons and singularities, but that does not necessarily mean that no time machine can ever be invented. It should be noted that Stephen Hawking has since changed his mind and decided that Kip Thorne's wormhole time machine might be possible if parallel universes exist.
There is another possibility when it comes to the universe protecting itself from paradox and it is nothing less than predestination. Consider the story of a woman who grew up in a time of war. As a young girl she was trained as a soldier and fought in many battles. In one particular battle she was injured by a gunshot to the shoulder. This forever caused her to have bad aim. One day in the future she joins a militant group that believes that the ideals of the past were wrong and they must return to the past to try and change the outcome of an important battle. The battle does not seem to go well for the future soldiers fighting the past soldiers. No matter what they do they cannot gain an advantage, and they retreat. As the woman is retreating she sees a young girl taking aim at the leader of her group. She fires at the girl hoping to hit her square in the head, but she misses. She strikes the young girl in the shoulder instead. It is then that she realizes that the young girl was her. Had she fired true she would have killed herself and created a paradox, but she could not fire true for she had bad aim due to the injury that she received from herself. In this scenario there is no paradox, but there is a disturbing ring of predestiny. The future caused the past. In fact the past required that the future already exist in order to avoid paradox.
Kip Thorne and his student, Michael Morris wished to study how paradoxes
might be averted by a thought experiment whereby one has a short wormhole that is a time
machine. Its two mouths are maybe only 1 foot apart in space and have a time difference of
only seconds. The experiment strives to find scenarios for a billiard ball entering one
mouth and travel backward in time to exit the other mouth and smack itself before entering
the first mouth. There were only a handful of solutions that did not create a paradox. All
of them involved the ball hitting itself in such a way as to have caused the collision in
the past. My favorite of these solutions is one where the ball travels straight between
the two mouths. Just before passing them by it is suddenly struck from the side by a
billiard ball emerging from the left mouth. This alters its trajectory so that it now
enters the right mouth and travels back just in time to emerge from the left mouth and
smack into itself thereby causing the earlier version to enter the other mouth. This
scenario has neither paradox nor causality violation. In fact, the effect was its own
cause. The future had to have existed already for the past to take the form that it did.
There is another line from a Star Trek: The Next Generation episode (Time's
Arrow) that involves such a scenario, where the Android, Data, remarks about the
finding of his head in an abandoned mine where it has apparently been for 400 years. It
seems that at some future point he will travel to the past where he will die. Of this he
says
``It has happened, it will happen.''
Finally, there is a scenario where an infinite loop of time, a mobius if you will, is created. Let us return to the scenario of my traveling to the past to try and avert the Kennedy assassination. We left me at the point where I was going to travel to the past to try and change history for the better. Another version of me has already done this once. He went back and stopped Kennedy from being assassinated. But what if that action did not have the consequences that I had hoped it would originally. What if by living President Kennedy becomes the cause of a nuclear war that devastates the Earth. Let us suppose that a version of me will be born in this dismal world lying in ruins. Let us also suppose that, like my alternate self, I come across a time machine and try to go back in time to change history. This time, what will I change? Because I know that Kennedy was the cause of the war that destroyed the human civilization I might now be inclined to go back in time and kill him before he has the chance. When would I do this, why November 22nd, 1963 of course, after all it seems that the time machine I have come across naturally has its other mouth in that time. I would become one of the conspirators that another version of myself will stop from committing this crime. In this way there are two histories which are co-existing and interacting with each other. The histories tie up in a loop that has no beginning or end, breaking the loop would be impossible because that would result in the paradox that we discussed before. The only other way to stop the loop would be to stop both versions of myself from time traveling, thereby never creating the loop.