manyworlds

Many-Worlds Interpretation (MWI) is an interpretation of quantum mechanics proposed by Hugh Everett III in 1957. It holds that the universal wavefunction never collapses. Instead, all possible outcomes of quantum events are realized in a branching multiverse, and measurement simply correlates observers with different components of the wavefunction.

Under MWI, the fundamental dynamics are the unitary evolution of the wavefunction as dictated by the Schrödinger equation. When a quantum system interacts with a macroscopic apparatus and environment, the combined state becomes entangled with many degrees of freedom. Decoherence rapidly suppresses interference between different branches, making them effectively independent and yielding objective “worlds” that follow quasi-classical histories.

Probability in MWI is interpreted as the measure or weight of branches rather than a single stochastic outcome. How the Born rule arises within this framework is a subject of debate; some approaches derive it from decision theory (Deutsch, Wallace), while critics argue the derivations are controversial or incomplete.

MWI contrasts with interpretations that invoke wavefunction collapse, such as Copenhagen. Initial reception was mixed, but the view gained traction with advances in decoherence theory and discussions of the ontology of many worlds. Related ideas include the relative-state formulation and connections to decoherent histories.

The interpretation remains controversial and is not experimentally distinguishable from other interpretations with current technology, since all yield the same empirical predictions. It has influenced philosophy of physics and popular discussions of quantum reality, often appearing in thought experiments like Schrödinger’s cat and discussions of cosmological multiverses.