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.
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
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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