Philosophy of Physics
Novikov Self-Consistency Principle: Why Time Travel Need Not Create Paradoxes
The Novikov self-consistency principle, developed by physicist Igor Novikov in the 1980s, holds that the only events that can occur near a {{closed timelike curve}} are globally self-consistent ones. The probability of a paradox-creating event is exactly zero, so time travel constrains action rather than producing contradictions.
Bootstrap Paradox: Information and Objects With No Origin
The bootstrap paradox, also called the ontological paradox, is a time-travel scenario in which an object, person, or piece of information causes itself, circulating in a {{causal loop}} with no point of creation. Unlike the grandfather paradox, it involves no logical contradiction, only an apparent absence of origin.
Causal Loop: Self-Caused Sequences of Events
A causal loop is a sequence of events in which one event is among the causes of another, which is in turn among the causes of the first. Such loops are theoretically permitted by spacetimes containing {{closed timelike curves}} and are the structure underlying the {{bootstrap paradox}}.
Closed Timelike Curve: Time Loops in General Relativity
A closed timelike curve (CTC) is a worldline in spacetime that returns to its own starting point, allowing an object to travel into its own past. Several exact solutions of {{general relativity}} permit CTCs, and they raise consistency and determinism problems that David Deutsch's quantum model attempts to resolve.
The Conservation-of-Information Objection: Why Actionable Knowledge Eats Its Own Origin
Sending information rather than an object through a time loop avoids physical wear but raises a different puzzle. A {{bootstrap paradox}} of pure information, like Beethoven's symphony with no composer, seems to violate an intuition about where complex structure comes from. Worse, actionable information such as lottery numbers tends to destroy the very timeline that produced it.
The Infinite-Wear Argument: Why Physical Objects Can't Survive a Bootstrap Loop
A self-consistent time-travel loop that carries a physical object forces that object to age on every pass through the loop. Because the loop has no first iteration, the wear compounds without bound, so the object would arrive already destroyed. This is a common informal argument for why a {{bootstrap paradox}} can carry information but not durable matter.
Looping vs Branching: Why You Can't Have Both Free Will and Genuine Time Travel
Time-travel models force a tradeoff. A single closed timeline avoids contradiction only by committing to absolute determinism, so apparent choice is illusory. {{Branching}} timelines preserve freedom but stop being real time travel, since you only ever affect an adjacent history. Closed causal structures can also be distributed across multiple branches as figure-eights or knots without dissolving.