On the Everett programme and the Born rule

TL;DR

This paper critically examines the derivation of the Born rule within the Everett interpretation, showing it requires an additional postulate equivalent to the projection postulate, thus questioning its natural emergence.

Contribution

It demonstrates that deriving the Born rule in the Everett interpretation necessitates an extra postulate, challenging claims of its purely unitary derivation.

Findings

Derivation of the Born rule requires an additional postulate.

Alternative postulates can lead to consistent but strange theories.

The projection postulate cannot be fully derived from unitary evolution alone.

Abstract

Proponents of the Everett interpretation of Quantum Theory have made efforts to show that to an observer in a branch, everything happens as if the projection postulate were true without postulating it. In this paper, we will indicate that it is only possible to deduce this rule if one introduces another postulate that is logically equivalent to introducing the projection postulate as an extra assumption. We do this by examining the consequences of changing the projection postulate into an alternative one, while keeping the unitary part of quantum theory, and indicate that this is a consistent (although strange) physical theory.