[ExI] Silence in the sky-but why?

[BillK]

On Sat, Sep 7, 2013 at 2:07 PM, Eugen Leitl  wrote:
> There are two issues with these assumption: the pioneer
> organisms are specialists, so the expanding wave front
> is different than the volume behind, which will have
> succession colonization waves, and then settle into
> a high-diversity equilibrium.
>
>

As you know, I am not keen on the idea of expanding wave fronts
colonizing the galaxy. :)
My main reason is (agreeing with Fermi) that considering galactic time
scales we should have been colonized already. We don't see this
happening in any other galaxy either, even nearby galaxies within a
few million light years distance.

The argument usually goes along the lines of -
crossing time (at one tenth the speed of light) of 300 years, add a
recuperation time of say ~ 700 years, each step in the expanding
wavefront takes ~ 1000 years, therefore to cross the entire galaxy
would take a few million years. Even less if you assume the wave front
doesn't stop for the ~700 years recuperation time.

I have found another objection to the wave front theory as well, quoting from -
Are Intelligent Aliens a Threat to Humanity? Diseases (Viruses,
Bacteria) From Space. Chandra Wickramasinghe, Ph.D., Centre for
Astrobiology, Cardiff University, UK .
Quote:
With the numerical values chosen in this example, our space colonisers
would need to have biological generation time (mean life-span)
considerably in excess of ours. Otherwise, we have to posit that the
potential predator embarks on a space voyage that benefits not its own
generation but several generations into the future. No example exists
on Earth where this model applies, either naturally in the living
world, or in a sociological context.
--------------

This paper suggests that galactic colonization is more likely along
panspermia lines. i.e distributing DNA by microbes and viruses,
leaving them to develop wherever they find a suitable environment.
This would be undetectable by us, but could produce life growing
throughout the galaxy.

BillK