“Two paradoxes are better than one; they may even suggest a solution” -Edward Teller
No discussion involving the feasibility of time traveling can be considered complete without a discussion involving the paradoxes involved. Let us consider the paradoxes commonly associated with time travel and examine whether it rules out the possibility of time traveling. Possible conjectures to overcome the problems of these paradoxes will also be discussed here.
Generally there are two main classes of paradoxes associated with time travel. First of which involves the alteration of previously recorded events or simply, the changing of the past. A good example of this is the famous Grandmother Paradox.
“What happens if you went back in time and killed your grandmother before she could conceive your mother? The result, you would have never been born. If so, you could have never have gone back in time, and so you could never have killed your grandmother.”
This sort of paradoxes might impose restrictions on the activities of the time traveler, but does not fundamentally rule out time travel in itself.
The second class of paradox involves the notion of causality, where every effect occurs as a result of its cause. Cause must precede effect. For instance:
“An art critic travels back in time to meet an artist whose work is greatly admired in the critic’s era. Upon arrival at the artist’s house, the critic views the existing pieces of art. He is disappointed. It seems that the artist has not attained the inspiration and greatness that he is famous for yet. Suddenly, the artist walks in. The critic tells the artist of his greatness in the future and shows the artist a book containing a collection of his future works. Unknowingly, the artist hides the book from the critic, forcing him to return to the future without his book and proceeds to spend the rest of his life mimicking the pictures found in the critic’s book.”
So where did the art work come from originally? Not the artist, because he stole it from the critic, and not the critic, because he got it from the artist. The art seemingly comes into existence from nowhere without any plausible explanation, hence the paradox. The implication is, therefore, since time travel negates causality, therefore it must be impossible.
There is much debate amidst the abounding theories regarding the implications of such paradoxes. Stephen Hawking proposed his chronological protection conjecture, suggesting that the laws of physics will always conspire to prevent time travel into the past.
However, physicists have come up with possible solutions to get around such paradoxes. One of which being the theory of parallel universes’. This concept means the existence of endless alternate versions of the universe. This conveniently does away with the paradoxes of time travel by allowing the time traveler to return to a parallel universe that is different to the universe which he started from. The time traveler simply creates an alternate universe by going back in time. Killing your grandmother before she could conceive your mother means that you would not have existed in this newly created universe. Nevertheless, you were still born in the universe from which you originated from. This is known as the many worlds” interpretation of quantum mechanics.
Novikov self-consistency conjecture has also been proposed as another solution to the time travel paradoxes. It states that if an event will lead to a paradox, then the possibility of an event occurring is zero. Using a mathematical model involving a billiard ball being launched into a wormhole, the paradox lies when the billiard ball exits the wormhole moments earlier in time to collide the incoming billiard ball, preventing it from ever entering the wormhole in the first place.
Novikov found that a given initial condition could result in many different paths for the billiard ball. He also found that the probability of the billiard ball doing something that is logically inconsistent (such as the above paradox) was zero. The ball cannot pass through the wormhole in such a way that it will prevent itself from passing through the wormhole, but that does not stop the ball from passing through the wormhole in the first place.
This means that a time traveler could presumably travel back to the past, but will be incapable of taking paradoxical actions. Such a conclusion might contradict the human concept of free will, but a human’s free will, however great, does not allow the laws of physics to be broken.
Time travel appeals, irresistibly, to the romantic soul of anyone who is human. In spite of the problems posed by the time travel paradoxes presented earlier, it does not fundamentally rule out time travel in itself. Have we already been visited by time travelers from the future? Who knows?