Predestination paradox
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A predestination paradox, also called a causal loop or 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 or her to travel back in time. This 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 born, thereby precluding his own travel to the past by canceling his own existence.
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, not changing it.
In physics, the Novikov self-consistency principle proposes that contradictory causal loops cannot form, but that consistent ones can. In a physical sense, a self-consistent causal loop of this kind is not actually a paradox because it produces a logically consistent result rather than a contradictory one. It is only perceived as a paradox because it goes against conventional expectations and assumptions about causality.
[edit] Examples
A typical example of a predestination paradox (used in an episode of The Twilight Zone) 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.
Another example (used in an episode of Futurama):
A man travels back in time and impregnates his grandmother. She would, as a result, give birth to the man's father, to whom will be born the man himself. This man would have to travel back in time in order to ensure his own existence.
A variation on the predestination paradox 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 all three 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 first 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 the second example, the man's very existence would be pre-determined by his time traveling adventure. This also raises the paradox of which came first — the time travel or his existence (see the closely-related Ontological paradox).
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.
[edit] Examples from fiction
Many fictional works have dealt with various circumstances that can logically arise from time travel, usually dealing with paradoxes. The predestination paradox is a common literary device in such fiction.
Prior to the use of time travel as a plot device, the self-fulfilling prophecy variant was more common, with two of the earliest and most famous examples being the ancient Greek legend of Oedipus, and the ancient Indian story of Krishna in the epic Mahabharata.
In the legend of Oedipus, it is prophesied that the baby Oedipus will one day kill his father and marry his mother. His father, Laius, attempts to circumvent the prophecy by abandoning the baby in the wilderness, where he was found by another King and Queen and raised as their son. Years later, Oedipus — unaware that he was adopted — learns of the prophecy and leaves home to avoid it. He kills a man and marries the widow, but does not learn until later that they are, in fact, his biological parents. The attempts to avoid fate result in the fulfilment of the prophecy.
In the story of Krishna in the epic Mahabharata, king Kamsa, afraid of a prophecy that predicted his death at the hands of his sister Devaki's son, had her cast into prison where he planned to kill all of her children at birth. After killing her first six children, Krishna took birth. As his life was in danger he was smuggled out to be raised by his foster parents Yasoda and Nanda in the Gokul village. As a young man, Krishna returned to his kingdom to overthrow his uncle, and Kamsa was eventually killed by his nephew Krishna. It was due to Kamsa's attempts to prevent the prophecy that led to it coming true.
