World Line

A world line is a single self-consistent history for the universe, extending from the beginning to the end of time. A very large, or infinite, number of these exist. World line is used as an alternative name for the distinct timelines, or worlds, which together form a continually branching quantum superposition of all possible histories. Contrary to a common misunderstanding, world lines are a feature of quantum mechanics. The physical theory used is based primarily on many-worlds quantum mechanics.

The rest of the wavefunction is not physical, but does still exist. It represents the possible worlds that can become real under certain circumstances. Crucially, in Steins;Gate universe it is said that only one of the many world lines exists physically at any one time:"World line — This term refers to an infinite number of possible worlds. However, these worlds do not exist in parallel. Only one exists at a given time. Everything that occurs along a world line, from the past to the future, has been predetermined. Thus, no matter how much you change the past along a single world line, the result will converge to the same outcome. — TIPS, Steins;Gate 0"The one world line that currently has a physical existence is often referred to as the active world line. The remaining world lines, which exist as possibilities in a quantum superposition, could correspondingly be called inactive world lines. Kurisu: "These superpositioned world lines aren’t parallel worlds, are they?"

''Suzuha: "No. At the very end, they’re just possible worlds that exist simultaneously." —'' Chapter 6, Steins;Gate