Frogwatch wrote:

My form of the Drake Equation:

N=ns*np*nl*nmc*ncmc*nil*nt

Whe

N = number of civilizations EVER in the galaxy
n = fraction of stars including brown dwarfs
np = fraction of stars having planets
nl = fraction having even single celled life
nmc = fraction having multi-cellular life
ncmc = fraction having complex multi-cellular life
nil = fraction having intelligent life
nt = fraction having technology

Plug in some numbers. I use 4 E 11 for the number of stars. .5 for
the number having planets, .5 for the number having single cellular
life, .03 for the number having multi-cellular life, .3 for the
fraction having complex multi-cellular life, .3 for the fraction
having intelligent life, and .1 for the fraction developing
technology. This gives only 2.7E7 civilizations EVER in the galaxy.

Now, I make an assumption that seems reasonable to me. That after
100,000 yrs, a civilizations technology will be so advanced that it
will ´disappear¡ from view. That is, it will either destroy itself
(unlikely) or will encode itself into the information contained in the
universe so it has no conventional physical form. You may not agree
with this but it makes sense to me given the rate of technical
advances.

There's another way a civilization could disappear from *our* view.
What if humans were to develop a fusion drive to be able to colonize the
Kuiper belt and then its technology advanced to the point the rockets
showed very little flame other than neutinos? Humans could colonize the
Ooort cloud. The cloud is so large there's some overlap star to star,
but the result could be a civilization that rarely goes deep enough into
a star's gravity well to deal with planets that have cleared their
orbits. A planet that has cleared its orbit does not have extra comets
to colonize.

Also try runing the math on a civlization that can colonize other star
systems. If the ships go 1% C and it takes a few centuries to build
industry in the colony system to the point it starts launching new
colonies it takes on the order of 10 million years to fill the galaxy.
That's 100 times your guess at the maximum lifespan so at most the
civilization fills 1% of the galaxy before it goes extinct. If your
number of 270 civilizations in the galaxy at the same time is accepted
that suggests that a sizable fraction of civilizations go their entire
existance without encountering any other civilization and that most as
so different by the time they encounter each other they might even
overlap across a wide range. If a civilization could last 10 million
years then we would already see flying saucers. Very low chances.

But combine the Oort cloud hypothesis with the numbers on volume and
expansion and it could happen that we are already embedded in an
interstellar civilization and we don't even know it because they are far
removed from stars.