[ExI] Silence in the sky-but why?

[Eugen Leitl]

On Sat, Sep 07, 2013 at 03:09:39PM +0100, BillK wrote:

> As you know, I am not keen on the idea of expanding wave fronts
> colonizing the galaxy. :)

I agree that this is not something what I would do. But what
we want is not relevant across time and space. Life will always
find a way.

In a sense a sterile universe wastes a lot of resources.
Consider the Moon. Perfect waste of atoms and joules, and
lovers be damned.

> My main reason is (agreeing with Fermi) that considering galactic time
> scales we should have been colonized already. We don't see this

If we were colonized, you would not be able to read this message.
There are far more ways of being dead than being alive, yet you're
observing yourself being alive. Probabilities mean nothing here,
as long as no external samples are available.

> happening in any other galaxy either, even nearby galaxies within a
> few million light years distance.

Try three orders of magnitude more.
 
> The argument usually goes along the lines of -
> crossing time (at one tenth the speed of light) of 300 years, add a

Once begun, expansion will ultimatively turn relativistic.
This means that probability of encountering subrelativistic
expansion is nil. They get hopelessly left behind across
time and across space. Your observation is heavily biased
towards very expansive observers. You will never encounter
any other kind, the chances are less than finding a needle
in a haystack.

> recuperation time of say ~ 700 years, each step in the expanding

Exponential processes ramp up quickly. A single stellar system
can become fertile almost immediately, and will autamplify by
many orders of magnitude. Not only neighbor systems will be
targeted. Solid state can cross intergalactic voids. Time is not important.

> wavefront takes ~ 1000 years, therefore to cross the entire galaxy
> would take a few million years. Even less if you assume the wave front

Way less.

> 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)

No biology will be part of expansion. It's all small-scale solid state.

> considerably in excess of ours. Otherwise, we have to posit that the
> potential predator embarks on a space voyage that benefits not its own

Pioneers are not predators. They just restructure everything that
no other life can emerge, and preexpansive life will be likely snuffed
out due to their metabolism. 

> generation but several generations into the future. No example exists
> on Earth where this model applies, either naturally in the living

Au contraire, species succession in volcanic island colonization is
a very good model, and happens all the time.

> world, or in a sociological context.
> --------------
> 
> This paper suggests that galactic colonization is more likely along

The paper is bunk. 

> panspermia lines. i.e distributing DNA by microbes and viruses,

There is no demonstrated interstellar panspermia transfer mechanism.
Intrasystem yes, intersystem, no.

> leaving them to develop wherever they find a suitable environment.
> This would be undetectable by us, but could produce life growing
> throughout the galaxy.

I'm not very interested in primitive life. Especially, since it's
undetectable. So let's rather look for our car keys under the 
streetlight.