MWI

MWI stands for the Many-Worlds Interpretation of quantum mechanics. Proposed by Hugh Everett III in 1957, it replaces wavefunction collapse with continuous, unitary evolution of the universal wavefunction. According to MWI, all components of a quantum superposition are realized, each in a separate, non-communicating branch or world. When a measurement occurs, the combined system of observer and measured system becomes entangled, and decoherence causes these branches to effectively evolve independently, giving the appearance of definite outcomes without invoking collapse.

The theory rests on the formalism of quantum mechanics: the wavefunction evolves according to the Schrödinger equation at all times, and probabilities reflect the observer’s subjective experience of being in one branch. The Born rule, which assigns probabilities to outcomes, is central to predictions but its derivation within MWI has been a topic of debate; proponents have offered derivations based on decision theory or symmetry (Deutsch–Wallace) or environment-assisted invariance (Zurek).

Critics argue that MWI multiplies ontological commitments and faces challenges such as the ontology of non-epistemic probability and the “preferred basis” problem. Proponents counter that decoherence provides a natural basis for branching and that the interpretation is empirically indistinguishable from other mainstream quantum interpretations, since all branches reproduce standard quantum statistics.

Historically, Everett’s ideas were initially controversial, later gaining influence through work by Bryce DeWitt and others who popularized the term “Many-Worlds.” Today MWI remains one of several leading interpretations of quantum mechanics, offering a deterministic, collapse-free account of quantum phenomena.