Published online by Cambridge University Press: 01 January 2022
Some have suggested that certain classical physical systems have undecidable long-term behavior, without specifying an appropriate notion of decidability over the reals. We introduce such a notion, decidability in μ (or d-μ) for any measure μ, which is particularly appropriate for physics and in some ways more intuitive than Ko's (1991) recursive approximability (r.a.). For Lebesgue measure λ, d-λ implies r.a. Sets with positive λ-measure that are sufficiently “riddled” with holes are never d-λ but are often r.a. This explicates Sommerer and Ott's (1996) claim of uncomputable behavior in a system with riddled basins of attraction. Furthermore, it clarifies speculations that the stability of the solar system (and similar systems) may be undecidable, for the invariant tori established by KAM theory form sets that are not d-λ.
I thank Wayne Myrvold, Howard Stein, William Wimsatt, Greg Lavers, Matthew Frank, Eric Schliesser, James Meiss, Norman Leibowitz, John Sommerer, Cristopher Moore, Jim Guszcza, Jill North, Amit Hagar, Joan Wackerman, David Malament, and the referees for comments on this and related efforts, the University of Western Ontario for the opportunity to present this research, Waid Parker for financial support, and Laurel Parker.