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Synopsis

Quantum mechanics has a problem with measurement: the very formulation of the theory seems to have ‘measurement’ or ‘observation’ built into it, and yet measurement and observation are physical processes like any other, and ought to be analyzable within physics rather than standing outside it. I will discuss the problem from a modern perspective, and consider how decoherence theory – our best current way to understand the quantum-classical transition – gives a satisfactory resolution of the problem of measurement, albeit one with disquieting philosophical consequences: taking it literally implies Everett’s infamous ‘many-worlds’ theory.