3rd, that's the question that has baffled scientists, academics and pub bores through the ages: What came first, the chicken or the egg? The futility of identifying the first case of a circular cause and consequence. The predestination paradox (also called either a causal loop or a causality loop) is a paradox of time travel that is often used as a convention in science fiction. It exists when a time traveller is caught in a loop of events that "predestines" him/her to travel back in time. Because of the possibility of influencing the past while time travelling, one way of explaining why history does not change is by saying that whatever has happened was meant to happen. A time traveller attempting to alter the past in this model, intentionally or not, would only be fulfilling his role in creating history as we know it, not changing it. The predestination paradox is in some ways the opposite of the grandfather paradox, the famous example of the traveller killing his own grandfather before his parent is conceived, thereby precluding his own travel to the past by canceling his own existence.
A dual example of a predestination paradox is depicted in the classic Ancient Greek play 'Oedipus'. Laius hears a prophecy that his son will kill him. Fearing the prophecy, Laius pierces Oedipus' feet and leaves him out to die, but a herdsman finds him and takes him away from Thebes. Oedipus, not knowing he was adopted, leaves home in fear of the same prophecy that he would kill his father and marry his mother. Laius, meanwhile, ventures out to find a solution to the Sphinx's riddle. As prophesied, Oedipus crossed paths with Laius and this leads to a fight where Oedipus slays Laius. Oedipus then defeats the Sphinx by solving a mysterious riddle to become king. He marries the widow queen Jocasta not knowing she is his mother.
A typical example of a predestination paradox (used in The Twilight Zone episode "No Time Like the Past" is as follows: A man travels back in time to discover the cause of a famous fire. While in the building where the fire started, he accidentally knocks over a kerosene lantern and causes a fire, the same fire that would inspire him, years later, to travel back in time.
A variation on the predestination paradoxes which involves information, rather than objects, traveling through time is similar to the self-fulfilling prophecy: A man receives information about his own future, telling him that he will die from a heart attack. He resolves to get fit so as to avoid that fate, but in doing so overexerts himself, causing him to suffer the heart attack that kills him. In both examples, causality is turned on its head, as the flanking events are both causes and effects of each other, and this is where the paradox lies. In the second example, the person would not have traveled back in time but for the fire that he or she caused by traveling back in time. Similarly, in the third example, the man would not have overexerted himself but for the future information he receives. In most examples of the predestination paradox, the person travels back in time and ends up fulfilling their role in an event that has already occurred. In a self-fulfilling prophecy, the person is fulfilling their role in an event that has yet to occur, and it is usually information that travels in time (for example, in the form of a prophecy) rather than a person. In either situation, the attempts to avert the course of past or future history both fail.
