Everettian theory as pure wave mechanics plus a no-collapse probability postulate

Abstract

Proposed derivations of the Born rule for Everettian theory are controversial. I argue that they are unnecessary but may provide justification for a simplified version of the Principal Principle. It’s also unnecessary to replace Everett’s idea that a subject splits in measurement contexts with the idea that subjects have linear histories which partition (Deutsch in Int J Theor Phys 24:1–41, 1985; The Beginning of Infinity. Allen Lane, London, 2011; Saunders and Wallace in Br J Philos Sci 59:293–305, 2008; Saunders, in: Saunders, Barrett, Kent, Wallace (eds) Many worlds? Everett, quantum theory, and reality, Oxford University Press, Oxford, pp 181–205, 2010; Wallace in The emergent multiverse, Oxford University Press, Oxford, 2012, Chapter 7; Wilson in Br J Philos Sci 64:709–737, 2013; The nature of contingency: quantum physics as modal realism, Oxford University Press, Oxford, forthcoming). Linear histories were introduced to provide a concept of pre-measurement uncertainty and I explain why pre-measurement uncertainty for splitting subjects is after all coherent, though not necessary because Everett’s original fission interpretation of branching can arguably be rendered coherent without it, via reference to Vaidman (Int Stud Philos Sci 12:245–66, 1998), Tappenden (Br J Philos Sci 62:99–123, 2011), Sebens and Carroll (Br J Philos Sci 69:25–74, 2018) and McQueen and Vaidman (Stud Hist Philos Mod Phys 66:14–23, 2019). A deterministic and probabilistic quantum mechanics can be made intelligible by replacing the standard collapse postulate with a no-collapse postulate which identifies objective probability with relative branch weight, supplemented by the simplified Principal Principle and some revisionary metaphysics.