Aliens need a lot motime to find us

On Jan 22, 2007, at 02:28 UTC, Alex Michael Bonnici wrote:

> "So, where is everybody?" Nobel laureate Enrico Fermi reportedly
> quipped to fellow physicists in 1950, when discussing why we haven't
> seen any signs of alien civilisations if, as many believe, our galaxy
> is teeming with life. Now, a maths model may have an answer to
> Fermi's paradox. Rasmus Bjork of the Niels Bohr Institute in
> Copenhagen has calculated that eight probes travelling at a tenth
> of the speed of light and each capable of launching up to eight sub
> -probes would take about 100,000 years to explore a region of space
> containing 40,000 stars.

..quoting a bit further: "When Bjork scaled up the search to include
260,000 such systems in our galaxy's habitable zone, the probes took
almost 10 billion years - three-quarters the age of the universe - to
explore just 0.4 per cent of the stars"

So, let's review: it takes ridiculously long to explore the whole
galaxy with 64 non-replicating probes. ...Umm, OK... but the relevance
of this is not clear to me. In real life, we wouldn't be talking about
a cilivization sending out eight probes (each with eight sub-probes)
and then sitting on their hands for ten billion years. We'd be talking
about cilivizations expanding from star to star. This is a
population-growth situation, not a fixed-number-of-probes problem. The
growth is basically exponential, until the galaxy starts getting full.
A few hundred million years is all it takes to reach that point.

It amazes me that otherwise respectable scientists (like Drake) and
mathematicians (like the Bjork guy) continue to make such basic
mistakes. They need to get some friends in the ecology department,
where population dynamics have long been understood.

Best,
- Joe

Joe Strout -- joe@...