Worldly Patterns: Emergence, Functionalism and Pragmatic Reality

Abstract

David Wallace's ``The Emergent Multiverse" is the most recent complete presentation of a version of Everett's theory of quantum mechanics that has attracted much scientific activity in the past decade. I present a brief sketch of Wallace's solution to the measurement problem, arguing that the many-worlds interpretation is not as far-fetched as it is often conceived to be. Taking the wavefunction as the fundamental ontology, it claims to solve the measurement problem by recognizing certain (quasi-classical) patterns of the wavefunction in 3N-dimensional configuration space that functionally behave like the (classical) configuration-space pattern of a N-particle system described by classical mechanics. In this sense, structures within the universal wavefunction are identified with classical `worlds' at the coarse-grained level. I highlight two elements of this work: the role of emergence and the functionalist framework that Wallace imports through what he calls `Dennett's criterion'. This criterion appeals to the virtue of usefulness (in the form of predictability and explanatory power) as a criterion for the reality of `patterns'.

It is shown that, due to decoherence, quasi-classical worlds emerge weakly, similar to that of the emergence of thermodynamic temperature from statistical physics, in the sense that they are autonomous and unexpected with respect to the lower-level domain (as opposed to strong emergence of some insuper et supra high-level ontology). However, the use of Dennett's criterion obscures this result, laying bare some philosophical issues, which we address over three axes of distinctions: (i) the objective/subjective-axis, (ii) the quantitative/qualitative-axis within the framework of intertheoretic reduction and (iii) the ontological/epistemological-axis.

First, Daniel Dennett's `real patterns' are compared to Wallace approach to patterns. Then, I point out (i) an analogy with Bas van Fraassen's idea of causal patterns that become salient due to pragmatic explanation, namely that in the context of a pragmatic goal the quasi-classical pattern is made salient over other objectively existing, non-classical, patterns. I conclude that in the absence of human goals there is no reason to regard the quasi-classical pattern as `more real' than other patterns. In analogy to Dennett's `intentional stance', Wallace is committed to a `classical stance', equivalent to breaking Hilbert space democracy of bases.

Although Wallace's version is a weaker one, his (ii) relations between theories bare resemblance to the reductionist program, and I argue that, next to quantitative deduction, additional conceptual `bridge principles' à la Ernest Nagel are needed. The appeal to usefulness as a criterion for reality (iii), I claim, is not necessary to solve the measurement problem itself, but has the further (unwarranted) goal of establishing `real' worlds. I spell out a solution to the measurement problem, the many-minds theory, which solves the problem along the same lines as The Emergent Multiverse, with the exception that the - although the quantum world itself is real - the classical worlds with definite properties are beliefs in the superposed brains of observers.