From: Hal Finney (hal@finney.org)
Date: Mon Jun 10 2002 - 11:54:44 MDT
I haven't had a chance to read all of the papers, but I've looked at the
first few paragraphs of some and skimmed others. Many of them discuss
the Fermi paradox but they mostly take a curiously nearsighted view.
The Fermi paradox seems to be generally expressed in galactic terms.
Why hasn't our galaxy been taken over by aliens? With all the stars in
the galaxy, surely one or more besides our own should hold intelligent
life, which can then sweep through the galaxy in a million years or less.
Nick's paper looks at the local supercluster and implicitly assumes
that it is unoccupied.
But really, the Fermi paradox is worse than this, in that it goes beyond
one galaxy. What about all the other galaxies we can see? Are they
devoid of intelligent life as well?
The local super-group constains 3 large galaxies: our own Milky Way,
the Andromeda galaxy, and M33, plus about 20 dwarf galaxies, all within 3
million light years. These are all close together, only a few diameters
apart, so if life had spread through any of them, it would have come
here, too. The Virgo Cluster contains 2500 galaxies within 60 million
light yeqrs. 60 million years is still a small fraction of the age of
the universe. If an expansive technological civilization spread out from
any galaxy in the Virgo cluster there is an excellent chance it would be
here by now. Or at least we would see evidence of its expansion through
the cluster.
And there are billions of galaxies visible in the universe. Of course
not all of these have been studied closely, but surely tens of thousands
have had some degree of detailed examination and classification. We need
to extrapolate the Fermi paradox and ask the question: which of these
galaxies have been taken over by intelligent life?
At one extreme, the answer is: none. Not one of the billions of galaxies
in the visible universe has had intelligent life spread through it and
make modifications, capture energy, and so on. Every galaxy we see
is basically as un-engineered as our own. Assuming that we do proceed
to go out and begin making changes to our galaxy, and then spread out
from there to other galaxies, we will be the only species in the entire
visible universe doing so.
That raises the Fermi paradox to a considerably higher degree! It's bad
enough to assume that there are no other civilizations in our own galaxy,
but in billions of other galaxies? It becomes a much sharper paradox,
harder to explain.
The other alternative is that although our own galaxy is not alive (that
is, it is in its natural state and has not been extensively modified by
intelligent life), that is not true for all of the galaxies we can see.
At least some of them have been taken over by life. Some of the galaxies
we see out there are living, and some are dead (or fallow).
This raises the question of what we would expect to see if life is
spreading through space. What would a living galaxy look like?
First, I think we might expect that the living region would be larger
than a galaxy and be roughly spherical in size. It seems likely that
life can spread through a galaxy in a very small amount of time on
an astronomical time scale. So we would not be too likely to observe
many galaxies in the middle of a conversion process. However, having
converted one galaxy, life would naturally go on to the next. And the
speed of light would constrain this expansion. Hence we would expect
to see a spherical region, whose size is based on the time since life
began its expansion, times the speed of the expansion. Galaxies within
that region would look fundamentally different from galaxies outside it.
How would the galaxies look? One possibility is that the stars would be
converted entirely into Dyson spheres or the equivalent, with as much
of the stellar output as possible being captured. An entirely Dyson
sphere'd galaxy would presumably be invisible in visible light and only
visible in long infrared. So what we would see through our telescopes
would be an apparently empty void of perhaps hundreds of millions of light
years in size, surrounded by a relatively normal arrangement of galaxies.
If we are able to observe in long infrared wavelengths, we would then see
that the interior of this region was radiating immense amounts of energy
at low temperatures, perhaps just barely above the cosmic background
temperature of about 3 K.
In summary we would expect the universe to be divided into living and
non-living regions. The living regions would be spherical and growing
at some substantial fraction of the speed of light. Galaxies within the
living regions should look very different from those outside of them.
We would be in a non-living region, and are about to begin our own process
of seeding a conversion. In a few hundred million years we will be at
the center of a newly formed bubble of living galaxies, similar to the
others which (perhaps) exist. More speculatively, the living regions
would contain dark galaxies whose radiation was entirely in the very
long infrared to microwave region.
I don't know whether this prediction can be reconciled at all with the
observational record, but it represents a midpoint between the extreme
Fermi scenario where we are the only intelligence in the Universe, and
the explanations of the Fermi paradox which somehow excuse the absence
of visible expansion.
Hal
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