Quentin Anciaux writes: > It has been said on this list, to justify we are living in "this" reality and > not in an Harry Potter like world that somehow "our" reality is simpler, has > higher measure than Whitte rabbit universe. But if I correlate this > assumption with the DA, I also should assume that it is more probable to be > in a universe with billions of billions of observer instead of this one. > How are these two cases different ?
I would answer this by predicting that any universe which allows for a substantial chance of billions of billions of observers would have to be much more complex. It would have a larger description, either in terms of its natural laws or of the initial conditions. Aside from the DA, we have another argument against the fact that our universe is well suited for advanced civilizations, namely the Fermi paradox: that we have not been visited by aliens. These two are somewhat similar arguments, the DA limiting civilization in time, and Fermi limiting it in space. In both cases it appears that our universe is not particularly friendly to advanced forms of life. The empirical question presents itself like this. Very simple universes (such as empty universes, or ones made up of simple repeating patterns) would have no life at all. Perhaps sufficiently complex ones would be full of life. So as we move up the scale from simple to complex, at some point we reach universes that just barely allow for advanced life to evolve, and even then it doesn't last very long. The question is, as we move through this transition region from nonliving universes, to just-barely-living ones, to highly-living ones, how long is the transition region? That is, how much more complex is a universe that will be full of life, compared to one which just barely allows for life? We don't know the answer to that, but in principle it can be learned, through study and perhaps experimental simulations. If it takes only a bit more complexity to go from a just-barely-living universe to a highly-living one, then we have a puzzle. Why aren't we in one of the super-living universes, when their complexity penalty is so low? OTOH if it turns out that the transition region is wide, and that you need a much more complex universe to be super-living than to be just-barely-living, then that is consistent with what we see. We are in one of the universes in the transition region, and in fact so are most advanced life forms. The relative complexity of super-living universes means that their measures are low, so even though they are full of life, it is more likely for a random advanced life form to be in one of the marginal universes like our own. In this way the DA is consistent with the fact that we don't live in a magical universe, but it implies some mathematical properties of the nature of computation which we are not yet in a position to verify. Hal Finney