Re: A _Rare Earth_ review (was: Scientific American article on GHZ)

From: Robert J. Bradbury (
Date: Mon Oct 29 2001 - 10:41:22 MST

Eliezer commented:
> Is anyone looking for a hypothesis under which intelligent life
> would happen, not once in a galaxy, but once in a Universe,
> or once in every 10^8 Universes?

Why would you care if it happens in other Universes? Are you
assuming inter-universe travel is possible?

> Because a rarity of once in a galaxy isn't enough to explain
> the Fermi Paradox, not nearly.

I assume you are saying this because intergalactic travel is
feasible. Without wanting to startup a whole long discussion
on the topic (because I should really finish the papers that
explain it clearly and those papers are 6 levels down in
my paper push down stack), intergalactic travel suffers
even more than interstellar travel from the time delay
effect. Looking at the LMC, when I happen to be in the
Southern Hemisphere, I have no idea whether or not it
is still there. It could be completely colonized by now,
in which case I should only plan to colonize it if I'm
willing to take it by force. The only real way I can
see for intergalactic colonization to occur is if you
wait for the galaxies to collide so your information
about its "state" is up-to-date. In which case colonization
doesn't occur at velocities limited by some significant
fraction of the speed of light but by the velocities of
galaxies (as well as the vectors that cause them to collide)
That has to be much slower, and will slow down further as
the universe expands.


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