In talk.origins John Wilkins <j.wilkins1@uq.edu.au> wrote:

Paul J Gans <gans@panix.com> wrote:

In talk.origins John Harshman <jharshman.diespamdie@pacbell.net> wrote:

...

Yes, one solution would be for all civilizations to render themselves
undetectable very soon after becoming detectable. This assumes they 
don't go in for travel or communication, and never make noticeable 
changes to their habitat (like Dyson spheres and such). It seems to me
that this assumption would require humans to be a very unusual sort of
intelligence, because we're going to go in for communication and travel
as soon as we figure out how, if we don't collapse first.

Other civilizations might well be signalling us like mad using
techniques we've not yet invented.

Or techniques we have abandoned? Semaphores?

Or obviously artificial signals such as the ones that begin:

  "I am Mr. Harson Gumbaw, nephew of the reigning oligarch
   of Obway.  I would like you to join me in a business venture
   that will make us both rich..."

-- 
   --- Paul J. Gans