Causal Loop: Self-Caused Sequences of Events

A causal loop, in the context of time travel and general relativity, is a sequence of events in which an event is among the causes of a later event that is itself among the causes of the first event. Causation runs in a closed circle, so the events have no external first cause. The term is often used interchangeably with bootstrap paradox and ontological paradox, since both describe objects or information that appear self-existing with no determinable origin. Causal loops are theoretically possible within general relativity because some exact solutions of Einstein's field equations contain closed timelike curves, paths through spacetime that return to their own starting point. A causal loop differs from a consistency paradox such as the grandfather paradox: a consistency paradox involves changing the past and generates a logical contradiction, whereas a causal loop accepts that events simply cause themselves, presenting a metaphysical rather than logical challenge. The Novikov self-consistency principle, proposed by physicist Igor Novikov, addresses how such loops can occur without contradiction: only globally self-consistent sequences of events are permitted near closed timelike curves, so whatever a time traveler does must already have been part of history. Fictional illustrations include Heinlein's "All You Zombies" and the 1980 film Somewhere in Time, in which an object passes through time with no traceable origin. See Bootstrap Paradox: Information and Objects With No Origin.