Quantum Foundations Podcast Deriving probability in quantum many-worlds with Dr Tony Short
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Feb 19, 2026 Tony Short, a theoretical physicist at the University of Bristol who studies quantum foundations, explains why a universal Schrödinger evolution leads to branching worlds. He outlines three simple assumptions that connect branching, decoherence and conserved weights to derive Born-rule probabilities. Short also discusses swap arguments, technical extensions for irrational amplitudes, and how these ideas fit into broader foundations research.
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Schrödinger-Only View Produces Branching
- Many-Worlds treats universal evolution as only the Schrödinger equation without special measurement rules.
- Branching appears as different portions of the single quantum state where copies of observers see different outcomes.
Determinism vs. Experienced Randomness
- Many-Worlds is deterministic at the fundamental level and contains no inherent randomness.
- Apparent randomness arises because individual copies experience only one branch, so outcomes 'feel' random to each copy.
Decision Theory Vs. Historical Explanation
- Decision-theory approaches (Deutsch/Wallace) derive cautious betting rules but focus on forward-looking rational action.
- Tony Short wanted an account that explains historical experimental frequencies, not just future betting behavior.