[Undecidability in] natural systems

Undecidable questions arise even in some traditional classes of models for natural systems. For example, in a generalized Ising model (see page 944) for a spin system the undecidability of the tiling problem implies that it is undecidable whether a given energy function leads to a phase transition in the infinite size limit. Somewhat similarly, the undecidability of equivalence of 4-manifolds implies undecidability of questions about quantum gravity models. In models based both on equations and other kinds of rules the existence of formulas for conserved quantities is in general undecidable. In models that involve continuous quantities it can be more difficult to formulate undecidability. But I strongly suspect that with appropriate definitions there is often undecidability in for example the three-body problem, so that the questions such as whether one of the bodies in a particular scattering process will ever escape to infinity are in general undecidable. In biology formal models for neural processes often involve undecidability, so that in principle it can be undecidable whether, say, there is any particular stimulus that will lead to a given response. Formal models for morphogenesis can also involve undecidability, so that for example it can in principle be undecidable whether a particular organism will ever stop growing, or whether a given structure can ever be formed in some class of organisms. (Compare page 407.)