There has been a huge amount written about the Fermi Paradox (why are there no aliens) over the years, and I don't want to reiterate that here. You can come up with scenarios in which intelligent life is common but where they just aren't visible, but IMO such explanations are not very natural. Instead I propose that for the purpose of our discussion here, we accept the apparent fact that there are no other intelligent life forms within the visible universe. Then let us consider the implications with regard to the All Universe Hypothesis (AUH), which says that all universes exist.
This observation points to the fact that with our laws of physics, the evolution of intelligent life is extremely unlikely. The question is, why? Not, why do our laws of physics make it hard for life to form, but why do we live in a universe whose laws of physics have this property? Presumably, there are universes whose laws make life essentially impossible. For example, they may be completely static, or equally bad, utterly chaotic. But on the other extreme, there must exist universes where intelligent life is common. At a minimum, we could create a such a universe in an ad hoc way by letting it be born full of intelligent life via forced initial conditions. And probably there are other laws of physics which would be much more congenial for the formation and sustenance of intelligent life than our own. So we have some universes which are full of life, others which are devoid of life, and others where there is a chance for life to form but it is relatively small. We appear to live in the third class. We talk about measure with regard to universes, and however it is defined, it seems that some such principle is needed to allow some universes to be more probable than others. Otherwise we have our flying rabbit paradox where the universe could suddenly stop being lawful, or could have arbitrary exceptions to lawfulness. Since there are more ways for things to go wrong than to go right, these exception-full universes would superficially be more numerous than those where the laws are universal. So there must be some property of the universal-law universes which makes it more probable for us to experience them than the others, and this is basically what we mean by measure. Universes with more measure somehow play a larger role in the multiverse and we are more likely to live in one of them. If universes with more consistent and uniform laws have greater measure, then this explains why we don't see exceptions like flying rabbits. However, it seems that the measure of a universe is not the only factor which should determine how likely it is to be observed; but in addition there should be a factor related to how many observers there are. The obvious case is for high-measure universes where observers are impossible. No one will observe such universes. This is the basic anthropic principle. But I would extend this principle to say that the probability of observing a universe is proportional to the product of its intrinsic measure and some factor relating to the number of observers in that universe. There are a few different ways this factor might work. The simplest would be to count the number of observers. A universe with similar measure but twice as many observers would be twice as likely to be experienced. Another possibility would be to use observer-moments. If two universes had the same number of observers, but in one they lived for twice as long as the other, then perhaps the second one would be twice as likely to be observed. Yet another alternative would be to base the factor on the fraction of the universe's total resources incorporated into observers, rather than just the number of observers. This would give a bonus to universes which were relatively efficient at creating observers, compared to universes which gained large numbers of observers merely be being inordinately large. The question of why we live in a sparsely populated universe, then, comes down to a comparison between the measure of a typical universe with many observers versus the measure of a typical universe with few. The former universes would get a large bonus factor for their many observers, while universes like ours don't have that. So for our observations to be consistent with the AUH, it must be that universes like ours have much larger intrinsic measure than universes with many observers. And since, as far as we can tell, our universe is not just sparsely populated, but extremely so, the measure differential in these two classes of universes must be extremely large. That is (turning to the Schmidhuber interpretation) it must be much simpler to write a program that just barely allows for the possibility of life than to write one which makes it easy. This is a prediction of the AUH, and evidence against it would be evidence against the AUH. On the face of it, this prediction doesn't seem too plausible to me. Of course, no one has ever written a program which evolves intelligent life, so we don't really know. But our initial explorations towards artificial life seem to indicate that it's not particularly difficult to achieve model universes just swarming with tiny and unintelligent replicators. Whether they could take the additional steps to become fully-fledged observers is an open question, one to which I suppose the AUH would have to predict the answer is no. Hal Finney
