Jeffrey A. Barrett and Peter Byrne, eds. The Everett Interpretation of Quantum Mechanics: Collected Works 1955–1980 with Commentary. Princeton, NJ: Princeton University Press, 2012. Pp. 392. $75.00 (cloth).

2013 ◽  
Vol 3 (2) ◽  
pp. 348-352
Author(s):  
Guido Bacciagaluppi
Keyword(s):  
Quantum Mechanics ◽  
Interpretation Of Quantum Mechanics ◽  
Everett Interpretation ◽  
Princeton University

scholarly journals Branch-counting in the Everett interpretation of quantum mechanics

2021 ◽  
Vol 477 (2255) ◽  
Author(s):  
Simon Saunders
Keyword(s):  
Quantum Mechanics ◽  
Ad Hoc ◽  
Quantum Probability ◽  
Objective Probability ◽  
Theory Approach ◽  
Interpretation Of Quantum Mechanics ◽  
Many Worlds ◽  
Everett Interpretation ◽  
Counting Rule ◽  
Bose Einstein

A defence is offered of a version of the branch-counting rule in the Everett interpretation (otherwise known as many worlds interpretation) of quantum mechanics that both depends on the state and is continuous in the norm topology on Hilbert space. The well-known branch-counting rule, for realistic models of measurements, in which branches are defined by decoherence theory, fails this test. The new rule hinges on the use of decoherence theory in defining branching structure, and specifically decoherent histories theory. On this basis ratios of branch numbers are defined, free of any convention. They agree with the Born rule and deliver a notion of objective probability similar to naive frequentism, save that the frequencies of outcomes are not confined to a single world at different times, but spread over worlds at a single time. Nor is it ad hoc : it is recognizably akin to the combinatorial approach to thermodynamic probability, as introduced by Boltzmann in 1879. It is identical to the procedure followed by Planck, Bose, Einstein and Dirac in defining the equilibrium distribution of the Bose–Einstein gas. It also connects in a simple way with the decision-theory approach to quantum probability.

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