[meteorite-list] Contact! - OT - ish

[Matson, Robert]

Hi Mark,

> N = N* fp ne fl fi fc Fl   (The Drake Equation)

I've always enjoyed jiggering with the numbers in the Drake
equation; unfortunately, most of the parameters are completely
unknown and so whatever value you choose is a complete guess.

Here's my w.a.g. at parameter values (vs. yours in parentheses):

N* represents the number of stars in the Milky Way Galaxy
N* = 500 billion   (100 billion)
(Btw, that's American billion, not British billion).  The actual
number of stars in the Milky Way is certainly at least 200 billion,
and could be over a trillion.

fp is the fraction of stars that have planets around them
fp = 50%  (60%)

ne is the number of planets per star that are capable of sustaining life
ne = 0.1  (0.33)

fl is the fraction of planets in ne where life evolves
fl = 20%  (10%)

fi is the fraction of fl where intelligent life evolves 
fi = 1%   (5%)

fc is the fraction of fi that communicate
fc = 5%   (10%)

fL is fraction of the planet's life during which the communicating
civilizations live.
L = 5000 years   (L = 1000 years)

You didn't indicate the average lifetime of the planet, but reverse
engineering your answers suggests that you assumed 10 billion years
(roughly the earth's expected lifetime).  I guess planetary lifetime
is intimately tied to stellar lifetime, which of course varies a
great deal depending on star type.  Since the majority of stars in
the Milky Way are red dwarfs, I would heavily weight stellar (and
thus planetary) lifetime toward the red dwarf lifetime -- around
100 billion years.  So I'll say 50 billion years.  So you and I still
end up with the same fraction (5000/50 billion vs. 1000/10 billion).
fL = 1E-7   (fL = 1E-7)

N = 0.25  (N = 1)

So we're within an order of magnitude of each other.  The main factor
affecting the outcome is the lifetime of a communicating civilization.
Suppose that once a civilization becomes advanced enough to communicate,
it doesn't die until its star does?  Then fL could be a million times
greater...

--Rob