Re: Measure, Doomsday argument

From: Hal Finney <hal.domain.name.hidden>
Date: Mon, 20 Jun 2005 14:12:17 -0700 (PDT)

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
Received on Mon Jun 20 2005 - 18:06:04 PDT