Many-Worlds Interpretation
The Many-Worlds interpretation, originating with Everett's 1957 relative-state formulation, holds that the universal wavefunction always evolves unitarily -- there is no collapse; what looks like a single measurement outcome is one branch of an entangled superposition. It is one of several live interpretations that make identical predictions for standard quantum experiments.
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What it means
Hugh Everett's 1957 relative-state formulation removes the collapse postulate: the entire universe is described by a single universal wavefunction that always evolves unitarily under the Schrodinger equation.In a measurement, the apparatus and observer become entangled with the system, so the total state becomes a superposition of branches, each containing an observer who sees one definite outcome relative to their branch.Decoherence explains why branches rapidly stop interfering and behave as effectively separate 'worlds'.The Born-rule probabilities are the main foundational challenge for the view: deriving why outcomes occur with weights |c_i|^2, rather than postulating it, remains actively debated (with decision-theoretic and other proposed derivations).Many-Worlds adds no new dynamics -- it is standard unitary quantum mechanics taken as universally valid.Like Copenhagen, it is an interpretation, not a distinct theory: for standard quantum experiments its predictions are identical to Copenhagen's and other mainstream interpretations', and no experiment to date has distinguished among them.Everyday analogy
Common misconceptions
- Experiments have NOT decided between interpretations -- Many-Worlds, Copenhagen, and other mainstream interpretations make identical predictions for standard quantum experiments, so no experiment to date favors one over another.
- Parallel worlds do NOT let you communicate across branches -- after decoherence, branches do not interact, so no signal, message, or resource can pass between them.
- Many-Worlds does not add new physical dynamics -- it is the standard unitary evolution taken as universally valid, with the collapse postulate removed, not an extra mechanism that 'creates' universes.
Key takeaways
- A single universal wavefunction always evolves unitarily -- no collapse postulate (Everett, 1957).
- Measurement entangles observer and system; each branch contains an observer seeing one definite outcome, and decoherence suppresses interference between branches.
- Deriving Born-rule probabilities is the view's central open challenge; predictions for standard experiments are identical to other interpretations, and no experiment has decided between them.
Check your understanding
According to the Many-Worlds interpretation, can information be sent between branches after a measurement?
- A.Yes, using entanglement
- B.Yes, but only forward in time
- C.No -- decohered branches do not interact, so no communication between branches is possible
- D.Only if the branches recombine spontaneously
Show the answer
Answer: C. No -- decohered branches do not interact, so no communication between branches is possible
Why: After decoherence, branches evolve without interfering with each other; the interpretation provides no mechanism for signaling between branches.
Builds on
Related concepts
Graded 2026-07-10 (human sign-off): open — no scientific consensus exists between interpretations; the physics predictions are identical (this is the honest core of the concept).
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