Novikov Self-Consistency Principle

The Novikov Self-Consistency Principle is a conjecture about the behavior of physical systems in the presence of closed timelike curves, proposed by Russian physicist Igor Novikov and collaborators. The principle states that any events occurring on a closed timelike curve must be self-consistent: a time traveler cannot alter the past in a way that produces logical contradictions, because the time traveler's actions are already part of the history they attempt to change.

Unlike the Chronology Protection Conjecture, which prohibits closed timelike curves outright, the self-consistency principle permits them but constrains their content through boundary conditions on the solution space. The principle has been explored in toy models involving billiard balls traversing time-travel loops, where self-consistent solutions exist alongside inconsistent ones that are dynamically excluded.

The principle raises deep questions about free will and determinism: if a time traveler is precluded from changing the past, are their actions truly volitional, or are they executing a pre-written script? The self-consistency principle suggests that causality is not a prohibition but a constraint — a boundary condition on what histories are physically realizable.