Re: poly: Fermi paradox and Percolation theory

From: Robin Hanson <>
Date: Tue Dec 29 1998 - 14:21:22 PST

Damien R. Sullivan,, writes:
> An interesting paper, which avoids the "We're alone",
> "Colonization is impossible", and "They're all avoiding us"
> solutions to the Fermi paradox.

Here is a relevant section from my filluniv.pdf paper:

The lack of alien visitors here on Earth now is striking, but
upon reflection is not terribly surprising. Even if the vast
majority of star systems in our galaxy have been colonized by
ETI, there are many reasons to expect a few exceptions. Our
star system might be part of an isolated zoo, for example, or
in an empty border zone.

Our failure to so far detect strong radio communications
between ETIs may also not be terribly surprising. After all,
we have sampled only a very small fraction of the space of
not-obviously unreasonable communication system parameters.
Our failure to detect communications directed at us may
perhaps be more surprising, if we assume Earth is of interest
to ETIs. For example, when we looked at 209 nearby stars
like our sun, we failed to hear anything like the equivalent
of Earth's currently most powerful transmitter (Arecibo Radar)
talking to us. At the ten nearest such stars, we failed to
hear any transmitter 500 times weaker.

On reflection, perhaps the most puzzling astrophysical
observation is the vast quantities of apparently unused
resources out there. If one thought most stars were colonized,
and if one expected mature stellar civilizations to make use of
a substantial fraction of the resources near their star, then
one might be surprised to learn that a survey of 230 observed
nearby stars like our sun found that less than one percent of
the starlight of each is being intercepted by local matter (at
least matter that reradiates near $300$K). This seems a
striking contrast to some predictions, such as by Barrow and
Tipler, of a complete exhaustion of material resources in the
wake of a colonization wave.

The great success of stellar physics also indicates that very
few stars have been much modified by local civilizations,
such as to extract mass. And we observe vast quantities of
apparently unused material in molecular and dust clouds. Such
apparently virgin mass may seem puzzling given than even now
we can sketch out engineering projects which might make use of
such resources. We can also see that civilizations surrounding
nearby stars dump very little nuclear waste into their stars,
and we can place limits on the use of powerful radars and
certain forms of starship drives.

Others before Landis had published on the idea that ETI could
be all around without being here on Earth. The zoo hypothesis
[Ball] was just the first. Turner published on the idea that
border zones between civilizations might have lots of empty

Hal Finney writes:
>> Any given colony will have a probability P of developing a
>> colonizing civilization, and a probability (1-P) that it will
>> develop a non-colonizing civilization.
>In our earlier discussions we assumed a correlation between the probablity
>of a colony's daughters colonizing and its parents. This led to an
>evolution-like model where there was selection for colonizing tendencies.
>... It might be interesting to explore various levels of correlation
>coefficient, between the 0 apparently assumed here and the 1 we assumed
>(as I understood the discussion).

Bainbridge published computer simulations where the probability
was correlated. He still had some empty spots, but he also saw
an evolutionary effect. He only simulated a small volume and I
think similar simulations in larger volumes would show much stronger
evolutionary effects. But there would likely still be some empty spots.

Robin Hanson
RWJF Health Policy Scholar FAX: 510-643-8614
140 Warren Hall, UC Berkeley, CA 94720-7360 510-643-1884
Received on Tue Dec 29 14:25:59 1998

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