A computation substrate has state and can change its state within (feedback) loops. An elementary particle suffices this definition: It has state because its pr

A computation substrate has state and can change its state within (feedback) loops. An elementary particle suffices this definition: It has state because its properties persist at least for some time, and it has feedback loops (interactions with other particles) that change its state. The more elementary particles locally affect each other, the more complex the state and feedback loops can be. What kinds of computation you can perform on such a substrate depends on the rules that govern the particles, in particular, how many feedback loops they allow to simultaneously affect a local arrangement of particles. It looks like there is an upper bound to this: No substrate can compute more kinds of functions than a Turing machine, which is a hypothesis that is closely tied to the conservation of energy and unitary (that the sum of the probabilities of all possible outcomes of a system equals 1). However, since matter decays, it is doubtful that anything around us can actually be as infinitely precise as the theoretical idea of the Turing machine.