Perimeter Institute will host a conference to mark the 50th Anniversary of Everett's paper proposing an interpretation of the universal quantum wave function.

Many Worlds interpretations aim to provide a simple, natural and realist understanding of unitary quantum theory without invoking wave function collapse or any special role for measurement. But, fifty years after Everett's original paper, there is still no consensus on whether they succeed. The meeting will bring together leading supporters and opponents of Many Worlds, in order to establish where the arguments currently stand and to enable sustained discussion and debate.

In 1957, Hugh Everett III proposed what he called a “relative-state formulation of quantum mechanics”, dropping the projection postulate, assuming that the universal wave function evolves unitarily, and arguing that the resulting theory has a natural interpretation consistent with our observations. Because Everett's strategy involves keeping all the terms in a macroscopic superposition – terms which seem to describe all the different possible outcomes of a measurement – it has often been described as a "many-worlds interpretation".

Sympathetic commentators have subsequently tried to flesh out Everett's original intuitions with new arguments, proposing several different "many-worlds interpretations" of quantum theory (many of which postulate some additional mathematical structure beyond the unitarily evolving wave function), and more recently "post-Everettian interpretations", which make essential use of technical work on decoherence and consistent histories to define the "many worlds".

Critics have focussed attention on several perceived problems with Everettian ideas, among them the apparent implicit choice of a preferred basis, the lack of precision in the definition of branching worlds, and the apparent appeal to some (possibly but not transparently correct) intuitions about the perceptions of conscious observers in superposition.

The role that probability plays in Everettian interpretations has also been much debated. Everettians have made several attempts to justify the use of the Born rule probabilities within an Everettian interpretation from some simpler assumptions: each of these in turn has attracted critical attention and purported refutation. A deeper question still is what it means, or could possibly mean, to ascribe a probability to experimental outcomes in an interpretation in which – in some sense – every possible outcome is realised.

Everettian ideas occupy a paradoxical position in modern theoretical physics. Much mainstream theoretical work assumes that Everett’s intuitions are basically correct: for example, essentially all of quantum cosmology is based on Everettian ideas. Yet the efforts of successive generations of Everettians and post-Everettians to amplify and clarify Everett’s ideas serve also to underline that there is still no complete consensus as to how, precisely, we should interpret the universal wave function and why, precisely, such an interpretation is consistent with our observations.