Introduction

In the July 1957 issue of the Reviews of Modern Physics: Hugh Everett iii put forward a new interpretation of quantum mechanics (Everett 1957). John Wheeler, Everett's thesis adviser, published, in the same issue, an accompanying paper supporting Everett's views (Wheeler 1957). Everett's aim was to cut through the fuzzy thinking displayed by many authors, some of them quite prominent, who in previous years had written incredibly dull papers on how they understood quantum mechanics. Everett's idea was simply to assume that quantum mechanics provides a description of reality in exactly the same sense as classical mechanics was once thought to do.

This is a shocking idea, for it leads to a multiplicity of “realities.” Few physicists in 1957 were prepared to accept it. And yet it can be shown to work. It is the purpose of this article to expand on Everett's original demonstration and to reveal the courage John Wheeler displayed in betting that his student was right.

Classical theory of measurement

System, apparatus, coupling

Our starting point is the standard theory of measurement, which we shall first examine classically. In its simplest form a measurement involves just two dynamical entities: a system and an apparatus. It is the role of the apparatus to record the value of some system observables. For this purpose system and apparatus must be coupled together for a certain period of time, which will be taken to be finite.