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Old August 27 2013, 10:22 AM   #26
Fleet Captain
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Location: At star's end.
Re: Whee are the aliens?

There are compelling arguments to believe that life is rare - at least in the Milky Way galaxy:

1. Abiogenesis.
What is the road from a bunch of chemicals to the "simplest" molecules that can replicate themselves halfway reliably?
Let's - VERY optimistically - assume that a specific chain of 100 chemical reactions are enough to create this "simplest" molecule.

Now - Darwinian selection has no part in creating this molecule; for Darwinian selection, you need self-replication, which you do not yet have.
Which leaves probability in charge. For a very rough approximation, calculate factorial 100. It gives a number so close to 0 as the chance of this "simplest" molecule emerging, that the chances are life on Earth is alone in the observable universe (and a huge chunk of the unobservable one).

Conclusion - abiogenesis is rare. VERY rare. Even if my factorial 100 approximation is exceedingly rough and even if there are hundreds of billions of Earth-like worlds in our galaxy, the chances are only ours ever gave birth to life.

2. The Fermi paradox.
This is based on a crucial and highly relevant fact:
An intelligent species, bound by the speed of light and various other engineering constraints (let's say, it can only reach 20% c) can colonise the entire Milky Way galaxy in 500.000+change years. And, once an intelligent species colonised other solar systems, this species (including its descendant species) is effectively immortal; no catastrophe can destroy it any longer.

The galaxy had enough heavy elements to evolve life since ~6-7 billion years ago. So, where are these ancient species? The entire galaxy should have been colonised may times over; they should be here, in our solar system - and everywhere else in the galaxy.
And then there are the robotic emissaries - von Neumann probes - of these species; also absent.

The immensity of time is FAR greater than the immensity of space (within our galaxy).

The pro 'life is common' argument relating to this fact tries to implausibly lump ALL these hypothetical species into a single planet-of-hats, adverse over billions of years to interstellar exploration/colonisation - choosing to ignore that even a single species will create many civilisations with different values and priorities during its history; that each species will be different from the others.

The Fermi paradox is NOT about whether intelligent species can send strong radio signals or about how their home planets look like.
It's about the fact that such intelligent species, if they exist, should already be in our solar system - and everywhere else in the galaxy.

They are not here; that's a very strong argument for their non-existence within the Milky Way galaxy.
"Let truth and falsehood grapple ... Truth is strong" - John Milton

Last edited by Edit_XYZ; August 27 2013 at 11:30 AM.
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