Many Worlds at 50 - 2007

This talk follows on from Wayne Myrvold\'s (and is based on joint work with Myrvold). I aim (and claim) to provide a unified account of theory confirmation that can deal with the (actual) situation in which we are uncertain whether the true theory is a probabilistic one or a branching-universe one, that does not presuppose the correctness of any particular physical theory, and that illuminates the connection between the decision-theoretic and the confirmation-theoretic roles of probabilities and their Everettian analogs. (The technique is to piggy-back on the existing body of physics-independent decision theory due to Savage, De Finetti and others, and to exploit the pervasive structural analogy between probabilistic theories and branching-universe theories in arguing for a particular application of that same mathematics to the branching case.) One corollary of this account is that ordinary empirical evidence (such as observed outcomes of relative-frequency trials) confirms Everettian QM in precisely the same way that it confirms a probabilistic QM; I claim that this result solves the Evidential Problem discussed by Myrvold. I will also briefly discuss the relationship between this approach and the Everettian \'derivation of the Born rule\' due to Deutsch and Wallace.

101 years ago William James wrote this about the Hegelian movement in philosophy: \'The absolute mind which they offer us, the mind that makes our universe by thinking it, might, for aught they show us to the contrary, have made any one of a million other universes just as well as this. You can deduce no single actual particular from the notion of it. It is compatible with any state of things whatever being true here below.\' With some minor changes of phrase---for instance \'mathematical structure\' in place of \'absolute mind\'---one might well imagine morphing this into a remark about Everettian quantum mechanics. This point, coupled with the observation that the Everett interpretation has been declared complete and consistent for the selfsame number of years that its supporters have been trying to complete it, indicate to me that perhaps the Everett approach is more a quantum-independent mindset than a scientific necessity. So be it, but then it should be recognized as such. In this talk, I will try to expand on these suspicions.