Everett and relative states

In this chapter I shall discuss briefly a number of the interesting developments – interpretational, theoretical, experimental – that have taken place in the foundations of quantum theory over the last few decades, where appropriate relating them to the work of Bohr or Einstein. While the topics considered will range well beyond the specific areas studied by John Bell, I think it is fair to say that it was the interest stimulated by his ideas that led to nearly all the work described.

This could not, though, apply to the very first ideas I discuss, which date from as early as 1957, when a PhD student at Princeton University, Hugh Everett, wrote a thesis titled The Theory of the Universal Wave Function [231]. A short version of this was published [232], and it was followed by a brief positive assessment [233] of Everett's ideas by John Wheeler, who had guided and encouraged him. (Sixteen years later, both of Everett's papers, and a number of related ones, were collected in a single volume [234].)

I shall now sketch his ideas. Till now, we have allowed wave-functions for microscopic systems, such as atoms or electrons, to be sums of wave-functions for highly distinct states, so that the corresponding properties of the systems – position, momentum and so on – do not usually have precise values, at least not till a measurement is made.