Which Universal Turing Machine? There are actually infinite sets of rules that can simulate all other rules, which is to be used for Solomonoff Induction. This choice affects the length of the hypotheses, and thus the probability we place on them. (doesn’t Eliezer Yudkowsky have something to say about this? he seemed to have an explanation of Occam’s Razor that might assume a specific Universal Turing Machine). In any case, different Universal Turing Machine rules don’t significantly change the hypothesis length relative to the compiler.