Abstract

A brief explanation of the meaning of the anthropic principle – as a prescription for the attribution of a priori probability weighting – is illustrated by various cosmological and local applications, in which the relevant conclusions are contrasted with those that could be obtained from (less plausible) alternative prescriptions such as the vaguer and less restrictive ubiquity principle, or the more sterile and restrictive autocentric principle.

Introduction

Having been asked to contribute a discussion of the anthropic principle for a colloquium on cosmology, Iwould start by recalling that although its original formulation [1] was motivated by a problem of cosmology (Dirac's) and although many of its most interesting subsequent applications (such as the recent evaluation [2] of the dark energy density in the Universe) have also been concerned with large scale global effects, the principle for which I introduced the term “anthropic” is not intrinsically cosmological, but just as relevant on small, local scales as at a global level. In retrospect I am not sure that my choice of terminology was the most appropriate, but as it has now been widely adopted [3] it is too late to change. Indeed the term “anthropic principle” has become so popular that it has been borrowed to describe ideas (e.g., that the Universe was teleologically designed for our kind of life, which is what I would call a “finality principle”) that are quite different from, and even contradictory with, what I intended. This presentation will not attempt to deal with the confusion that has arisen from such dissident interpretations, but will be concerned only with developments of my originally intended meaning, which I shall attempt to explain in the next section.