On Wed, 30 Jul 2003 00:22:24 -0400, "Tony Sivori"
<TonySivori@yahoo.com> wrote:

Mike Williams wrote:

They don't need to transport significant numbers at any one time between
the stars.

Why not? Do you think the Mayflower brought two Europeans to America, and
then no more? It didn't happen that way.

The Americas were already inhabited when Mayflower made its trip. The
native Americans were all descended from a few small bands that made
the trip over the Berings land bridge.

All you need to populate a planet is a few dozen individuals. If
you've got sperm banks, you don't even have to worry too much about
tailoring their diversity. If you've got artificial wombs, you don't
need to send _anyone_ to start a colony.

The problem with exponential population growth is that no finite amount
of real estate will be enough.

Not true. There are our old and ever-present companions; disease, famine,
natural disaster and war.

Those are things that happen when we run out of real estate and other
resources. They're a symptom of not having "enough".

Let's suppose that there's a civilisation whose numbers double every 50
years, and they have a technology that's capable of sending out 10
spaceships carrying two people each to the 10 nearest stars, with enough
technology to get a colony started.

Two people to the 10 nearest stars? The is no likelihood of finding even one
inhabitable planet in 10 solar systems, and if you did reach a livable
planet a colony of two is utterly doomed.

If you can build a starship capable of sustaining a colonizing
population during the centuries spent between stars, you can also
build habitats when you arrive. A "habitable system" in this case
means any star system with orbiting debris consisting of relatively
heavy elements.

A couple arrive and start breeding. After 50 years there's 4 aliens on
each new planet. After 100 years there's 8. After 500 years there's
1024.

You mean there are a few dying inbred drooling idiots.

Inbreeding only causes problems when the starting population has
deleterious recessive genes in it. If you carefully select your
starting population to be free of defects, then the children will be
fine.

Alternately, bring a sperm bank. You can pack lots of sperm into a
small freezer.

After 6000 years there's 10000 new colonies.

Your numbers sound like a pyramid scheme, and as we all know they *never*
reach their theoretical limits. Something else always stops them first. I.e,
you'll never get every available person to buy into any given scheme.

He's already mentioned the various limits they'll run into, and the
worst that they'll do is limit the expansion of the sphere of
inhabited systems to the maximum speed their colony ships are capable
of. Exponential reproduction slows down when you run out of space to
fill or resources to use - the exact situation that would be so
blatantly noticeable, as far as SETI is concerned.

So why aren't they here?

Because you seem to have greatly underestimated the odds and difficulties of
new colonies spreading across interstellar distances, surviving, and then
reacquiring high technology.

Interstellar is a snap, on these timescales. It could even concievably
be done by accident, by Oort cloud comet colonies.