Anthropic reasoning in multiverse cosmology and string theory

TL;DR

This paper critically examines the weak anthropic principle in multiverse and string theory, proposing a clearer formulation that avoids unwarranted assumptions and overreach in anthropic reasoning.

Contribution

It clarifies the ambiguity in the weak anthropic principle and advocates for a formulation that does not require additional assumptions like typicality.

Findings

WAP can be reformulated into two unambiguous versions.

WAP_2 requires a typicality assumption to make predictions.

WAP_1 directly implies our universe's observed properties without extra assumptions.

Abstract

Anthropic arguments in multiverse cosmology and string theory rely on the weak anthropic principle (WAP). We show that the principle, though ultimately a tautology, is nevertheless ambiguous. It can be reformulated in one of two unambiguous ways, which we refer to as WAP_1 and WAP_2. We show that WAP_2, the version most commonly used in anthropic reasoning, makes no physical predictions unless supplemented by a further assumption of "typicality", and we argue that this assumption is both misguided and unjustified. WAP_1, however, requires no such supplementation; it directly implies that any theory that assigns a non-zero probability to our universe predicts that we will observe our universe with probability one. We argue, therefore, that WAP_1 is preferable, and note that it has the benefit of avoiding the inductive overreach characteristic of much anthropic reasoning.