[ExI] against Many Worlds QT

[Damien Broderick]

http://arxiv.org/abs/0905.0624


One world versus many: the inadequacy of Everettian accounts of 
evolution, probability, and scientific confirmation

Authors: <http://arxiv.org/find/quant-ph/1/au:+Kent_A/0/1/0/all/0/1>Adrian Kent
(Submitted on 5 May 2009)
Abstract: There is a compelling intellectual case for exploring 
whether purely unitary quantum theory defines a sensible and 
scientifically adequate theory, as Everett originally proposed. Many 
different and incompatible attempts to define a coherent Everettian 
quantum theory have been made over the past fifty years. However, no 
known version of the theory (unadorned by extra ad hoc postulates) 
can account for the appearance of probabilities and explain why the 
theory it was meant to replace, Copenhagen quantum theory, appears to 
be confirmed, or more generally why our evolutionary history appears 
to be Born-rule typical. This article reviews some ingenious and 
interesting recent attempts in this direction by Wallace, Greaves, 
Myrvold and others, and explains why they don't work. An account of 
one-world randomness, which appears scientifically satisfactory, and 
has no many-worlds analogue, is proposed. A fundamental obstacle to 
confirming many-worlds theories is illustrated by considering some 
toy many-worlds models. These models show that branch weights can 
exist without having any role in either rational decision-making or 
theory confirmation, and also that the latter two roles are logically 
separate. Wallace's proposed decision theoretic axioms for rational 
agents in a multiverse and claimed derivation of the Born rule are 
examined. It is argued that Wallace's strategy of axiomatizing a 
mathematically precise decision theory within a fuzzy Everettian 
quasiclassical ontology is incoherent. Moreover, Wallace's axioms are 
not constitutive of rationality either in Everettian quantum theory 
or in theories in which branchings and branch weights are precisely 
defined. In both cases, there exist coherent rational strategies that 
violate some of the axioms.