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
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
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.