Probability in the Many-Worlds Interpretation of Quantum Mechanics

Howard Barnum

This unpublished 1990 preprint argues that a crucial distinction in discussions of the many-worlds interpretation of quantum mechanics (MWI) is that between versions of the interpretation positing a physical multiplicity of worlds, and those in which the multiplicity is merely psychological, and due to the splitting of consciousness upon interaction with amplified quantum superpositions. It is argued that Everett's original version of the MWI belongs to the latter class, and that most of the criticisms leveled against the MWI, in particular that it is illogical or incoherent, are not valid against such "psychological-multiplicity" versions. Attempts to derive the quantum-mechanical probabilities from the many-worlds interpretation are reviewed, and Everett's initial derivation is extended in an attempt to show that these are the unique possible probabilities. But there remains a challenge for proponents of the MWI: to show that their interpretation requires probabilities, rather than merely nonprobabilistic indeterminacy. A 2002 preface, revised in 2004, briefly discusses the extent to which I still agree with the claims in the paper. While its derivation of probabilities used, and failed to justify, noncontextuality, I still agree with the paper's general interpretation of the MWI, though not with the MWI itself.

Robert A. Van Wesep

Martijn Hidding

This thesis takes a look at the many-worlds interpretation (MWI) and discusses if it can become the leading interpretation of quantum mechanics. Firstly the Copenhagen interpretation is outlined and its issues relating to the measurement problem are discussed while taking recent insights in decoherence theory into account. Establishing that wave-function collapse still yields interpretational issues, I discuss the ideas of Hugh Everett, who proposed the many-worlds interpretation, and David Wallace who is a prominent advocate of the MWI. In particular I discuss and review Wallace's arguments in favor of viewing the wave-function and the many worlds realistically. The next part shortly focuses on the testability of the many-worlds interpretation and the philosophical and theoretical issues relating to probability in the MWI. I end by concluding that the no-collapse interpretations are preferable to the Copenhagen interpretation, and I argue in favor of a realistic view towards all of the worlds. Furthermore, I conclude that standard textbooks on quantum mechanics should be updated to introduce students to the insight that wave-function collapse isn't needed in forming a complete theory of quantum mechanics.

Van Wesep, A. Robert

Arthur I. Fine

Bruce S. Bennett, Moletlanyi Tshipa

Abstract The Many-Worlds Interpretation (MWI) is a theory in physics which proposes that, rather than quantum-level events being resolved randomly as according to the Copenhagen Interpretation, the universe constantly divides into different versions or worlds. All physically possible worlds occur, though some outcomes are more likely than others, and therefore all possible histories exist. This paper explores some implications of this for history, especially concerning causation. Unlike counterfactuals, which concern different starting conditions, MWI concerns different outcomes of the same starting conditions. It is argued that analysis of causation needs to take into account the divergence of outcomes and the possibility that we inhabit a less probable world. Another implication of MWI is convergent history: for any given world there will be similar worlds which are the result of different pasts which are, however, more or less probable. MWI can assist in thinking about historical causation and indicates the importance of probabilistic causation.