[Note that I take the liberty of replying only to the list, so that senders of earlier messages in the thread do not receive multiple copies of my messages.]

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David Barrett-Lennard writes: > An interesting idea. > > Where can I read a more comprehensive justification of this > distribution? Juergen Schmidhuber, who originated many of these concepts, has a number of articles on distributions. He is not a big fan of the UD but I think it is appropriate for what seems to me the simplest model of computing, Turing equivalent machines. He has a page on the UD at http://www.idsia.ch/~juergen/icmlkolmogorov/node2.html and if you look around his site you can find much more. Chaitin's work on Algorithmic Information Theory is related as well, http://www.cs.auckland.ac.nz/CDMTCS/chaitin/. He works with a measure of complexity based on program bit length that implicitly points to the UD. Chaitin's most famous result relates to the constant Omega, the probability that a random program will halt, where again the programs are implicitly selected according to the UD. > If a number of programs are isomorphic the inhabitants naturally won't > know the difference. As to whether we call this one program or lots of > programs seems to be a question of taste and IMO shows that "probability > calculations" are only relative to how one wants to define equivalence > classes of programs. The idea is that if all program strings are equally probable (admittedly a problematic assumption given that programs can be infinitely long), that implicitly leads to the universal distribution as a measure. This solves the problem you raised (about why the universe is lawful) in a very natural way. > I would expect that the probability distribution will depend on the way > in which we choose to express, and enumerate our programs. Eg with one > instruction set, infinite loops or early exits may occur often - so that > there is a tendency for simplistic programs. On the other hand, an > alternative instruction set and enumeration strategy may lead to a > distribution favoring much longer and more complex programs. Perhaps it > tends to complicate programs with long sequences of conditional > assignment instructions to manipulate the program state, without risking > early exit. Importantly such "tampering" doesn't yield a program that is > isomorphic to a simple one. We seem to have a vast number of > complicated programs that aren't reducible to simpler versions. This > seems to be at odds with the premise (of bits that are never executed) > behind the Universal Distribution. If I understand your concern, this has been shown to be not a problem. Any universal computer can simulate any other using a prefix of fixed size. Therefore the size of minimal programs for any two universal computers can vary by only a constant, and the UD's determined by these two computers will similarly agree to within a constant factor. Hal Finney P.S. In http://www.escribe.com/science/theory/m3666.html I discussed a way in which the UD might actually give us reason to expect the universe to be surprisingly unlawful.