**, which is**

*Everett and Evidence*by Hilary Greaves and Wayne Myrvold**chapter 9**of the July book of the month

*Many Worlds?: Everett, Quantum Theory, & Reality*edited by Saunders, Barrett, Kent, Wallace. We are discussing this book chapter-by-chapter, including a discussion for the introduction and the two transcripts. Click here for the organized linked list of each chapter's discussion.

**Please do not post in this topic until you have read chapter 9**.

***

What do you think of this chapter? What do you think of Greaves and Myrvold's ideas and arguments? If you chose mostly disagree or utterly disagree for the poll, then on what specifically do you disagree with them?

I very much like the way Greaves and Myrvold explain and attempt to address what they call the

*Everettian evidential problem*. This also meant making an argument regarding probability that does not assume Everettian Quantum Mechanics (EQM) or the Many World Interpretation (MWI). This sets it apart from the previous two chapters which both addressed probability in moderately different ways by the two Everettian sympathizers Saunders and Wallace. I had not considered the evidential significance of probability and the problems it arguably causes for EQM/MWI. I found there explanation of the apparent problem interesting and well-written and I found both their argument to nullify it and the paper as a whole to be well-structured and intriguing.

As with the other papers on probability, even though I do not accept EQM/MWI, I do not hold the probability issue against EQM. I think it is fine simply to reject the notion of a need to explain probability or assume any probability or chances exist beyond that stemming from everyday ignorance and assumptions of casual symmetry (e.g. that a die has even 'chances' of landing on any one side) which exist even in a deterministic universe. The Everettian evidential problem in a way could even be looked at -- philosophically speaking -- as a talking point of support for EQM since some of the assumptions that help support EQM such as the assumption that the universe is deterministic (i.e. that actual ontological probabilities based on truly random events do not exist) could be seen to solve the evidential problem. In other words, as a philosophical tactic it could be argued instead to simply say that indeed the issue of probability casts doubt on Quantum Mechanics all-together but not if one assumes the universe is deterministic. The authors in this chapter (and in slightly different ways in the previous two chapters) seemed to imply this point as another way of looking at it by emphasizing that they were only showing that EQM can explain probability at least as well as one-world interpretations. That of course leaves open what I am saying too which is that all Quantum Mechanics has that problem.

What do you think?

In any case, for a refresher, here is the abstract of chapter 9:

Hilary Greaves and Wayne Myrvold wrote:Much of the evidence for quantum mechanics is statistical in nature. Relative frequency data summarizing the results of repeated experiments is compared to probabilities calculated from the theory; close agreement between the observed relative frequencies and calculated probabilities is taken as evidence in favour of the theory. The Everett interpretation, if it is to be a candidate for serious consideration, must be capable of doing justice to this sort of reasoning. Since, on the Everett interpretation, all outcomes with non-zero amplitude are actualized on different branches, it is not obvious that sense can be made of ascribing probabilities to outcomes of experiments, and this poses a prima facie problem for statistical inference. It is incumbent on the Everettian either to make sense of ascribing probabilities to outcomes of experiments in the Everett interpretation, or to find a substitute on which the usual statistical analysis of experimental results continues to count as evidence for quantum mechanics, and, since it is the very evidence for quantum mechanics that is at stake, this must be done in a way that does not presuppose the correctness of Everettian quantum mechanics. This requires an account of theory confirmation that applies to branching-universe theories but does not presuppose the correctness of any such theory. In this paper, we supply and defend such an account. The account has the consequence that statistical evidence can confirm a branching-universe theory such as Everettian quantum mechanics in the same way in which it can confirm a non-branching probabilistic theory.