Ron Hale-Evans writes:
> My favourite answer to the Fermi Paradox has been that the aliens are
> using nearly-perfect compression or encryption for their radio signals
> (if they're using radio), and that's why all we can detect is noise.
>
> However, tonight another "answer" occurred to me. What if we're living
> in a finite simulation?

I don't know that multiverse concepts explain the Fermi paradox, but
they do cast it in a different light.

As Bruno points out, our first-person experiences could be created by
many different kinds of programs, corresponding to different "realities".
It could be that everything is pretty much as it seems.  Or perhaps we
are living in a simulation controlled by aliens, or our descendants,
or robots.  Or it's even possible that everything is an illusion and we
are in effect imagining it.  All of these possibilities contribute to the
measure of our experiences.  So in some sense it must be simultaneously
true that we are in a simulation, and that we are not in a simulation.
Both situations exist in the multiverse and both contribute to the
reality of our experiences.

The hard part of the Fermi question still remains.  It might be stated,
why is the universe seemingly so large and so empty?  In multiverse
terms, why is the measure of observers who live in large, empty universes
so large, compared to the measure of observers who live in universes
teeming with life?  For if the measure of the latter observers were much
greater than the measure of the former, we would be highly unlikely to
find ourselves one of that very small set of observers who see sparse
universes.

(Of course, I am skipping past the various conventional explanations that
have been offered which allow for the universe to in fact be full of life
but for it somehow not to be observable.  Those have not been generally
found to be convincing so we should focus on the hard part.  Also,
note that while I write "life" for short I really mean intelligent life.)

A while back I speculated as follows.  Presumably there are laws of
physics which would lead to very densely populated universes.  And we
know that there are laws that lead to very sparse universes, like
the ones we live in.  All universes exist; all laws are instantiated.

For various reasons many of us argue that universes with simpler laws
are likely to be more common, to have larger measure.  Now, we know that
if the laws are too simple, life cannot exist.  Trivial universes are
not living ones.  Presumably, as the laws get more complex, we pass a
threshold where life can start to exist.  But perhaps it is reasonable
to assume that we will first find laws where life can barely exist,
before we find laws where life is very common.  If so, then there is a
band of complexity where universes at the simple end of this band have
very sparse intelligent life, and universes at the complex end have very
dense intelligent life.

Then, to be consistent with our observations, we have to conclude that
this band is quite wide - that universes that are just barely complex
enough for life have much simpler laws than universes that are teeming
with life.  That is how we would explain the fact that we find ourselves
in one of the first kind.  Their boost from having simpler laws must
outweigh the increase in numbers of intelligent life forms in the more
complex universes.

I read that the universe is estimated to have about 10^23 stars.
A universe with a high density of intelligent life might therefore be
10^23 times more densely populated than ours.  This is about 2^75 times.
Therefore we would predict that the physical laws necessary to create
such a densely populated universe would be at least 75 bits longer than
the simpler laws of our own universe.

This is a prediction of multiverse theory as I interpret it.  If it should
turn out that there are very simple sets of laws that would create very
numerous observers, then that would contradict the theory in this form.

Hal Finney

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