Many-Worlds Interpretation — Wikipedia Summary

The many-worlds interpretation (MWI) is an interpretation of quantum mechanics asserting that the universal wave function is objectively real and that there is no wave function collapse. All possible outcomes of quantum measurements are physically realized in different “worlds”. Evolution of reality as a whole is rigidly deterministic and dynamically local. Also called the relative state formulation or the Everett interpretation, after Hugh Everett who first proposed it in 1957. Bryce DeWitt popularized the formulation and named it “many-worlds” in the 1970s.

Core Ideas

• The linear, unitary dynamics of quantum mechanics applies everywhere and at all times, describing the whole universe.
• Measurement is modeled as a unitary transformation (correlation-inducing interaction) between observer and object, without collapse postulate.
• Contrast with Copenhagen interpretation: no division between classical and quantum domains.
• The universe (multiverse) is a quantum superposition of increasingly divergent, non-communicating parallel universes (“Everett worlds”), each an internally consistent actualized alternative history.
• In modern versions, subjective appearance of collapse is explained by quantum decoherence. Under scrutiny of the environment, only pointer states remain unchanged; other states decohere into mixtures of stable pointer states (einselection) - Zurek.
• Similar to decoherent histories interpretation, but MWI treats all histories/worlds as real.

Preferred Basis Problem

• Everett/DeWitt originally gave measurements a privileged role in determining the basis. This was problematic (contradicts reductionist goal).
• Solved (per Saunders, Wallace) by incorporating decoherence: preferred basis = basis stable under environmental decoherence. Worlds are not part of fundamental ontology but of emergent ontology.
• Criticized as circular with probability derivations (Kent, Zurek, Baker).

History

• Originated in Everett’s Princeton PhD thesis “The Theory of the Universal Wave Function” under John Archibald Wheeler; published 1957 as “Relative State Formulation of Quantum Mechanics” (Wheeler contributed the title “relative state”; Everett originally called it “Correlation Interpretation”).
• Phrase “many-worlds” due to Bryce DeWitt, who popularized it in the 1970s.
• Precedent: Erwin Schrodinger’s 1952 Dublin lecture - “not alternatives but all really happen simultaneously” (earliest known reference to many-worlds per Deutsch).
• Zeh came to same conclusions before reading Everett, then built quantum decoherence theory based on these ideas (1970 paper).

Reception

• Initial reception overwhelmingly negative; ignored except by DeWitt. Bohr and collaborators completely rejected the theory. Everett left academia in 1957. Wheeler disavowed the theory in 1980.
• Strong advocate: David Deutsch (quantum computing, double slit explained by interference of photons across universes).
• Critics: Ladyman & Ross (equivocal), Penrose (oversimplified; gravity not accounted), ‘t Hooft, Peres, Stenger, Gell-Mann (rejected in favor of post-Everett/decoherent histories).
• Polls: 1991 Raub poll 58% of quantum cosmologists “yes”; Tegmark 1997 poll MWI second; 2011 Schlosshauer poll 6/33 endorsed MWI.

Speculative Implications

• Everything conceivable (consistent with physics) happens somewhere; freak events exponentially rare (Tegmark).
• Quantum suicide/quantum immortality thought experiments - most experts believe they would not work (worlds with surviving experimenter have lower measure).
• Carroll: human decision-making is classical process (neurochemistry), not quantum; weight of realized outcomes concentrated in a single branch.