lou@cadence.com (Lou Scheffer) wrote in
news:3a6089b6.0405012022.6070afd2@posting.google.com: 

You don't need a solar system scale transmitter - it's entirely within
the technology we have today.  50,000 ly = 5x10^20 meters, more or
less.  10 AU is roughly 1.4x10^12 m.  So you need a transmitter 3x10^8
wavelengths across to do this.  With a 1 cm wavelength, that's only
3000 km, so the array of transmitters could easily fit on any of our
continents.

Erf, you are right, of course.
I slipped a digit (three actually) in my calc on baseline length required.

The point i tried to make is even more valid than I thought: It is *easy* 
to focus a microwave transmission sufficiently so that one can transmit 
"louder" than your parent sun at interstellar distances, in a chosen 
frequency, without anything like prohibitive transmission power usage. As 
long as you know exactly where to point your transmitter at.

The only limitations in being an *active* participant in SETI, rather 
than a passive listener, are:
Finding a target that looks a likely candidate.
Waiting the rather huge timedelay while your speed-of-light message crawls 
to it.
Ensuring the content of your transmission is unmistakeably artificial.

Meaning: There are a heap of logistical issues, especially time-related 
ones, that hamper a practical SETI program. But there are NO fundamental 
physics preventing your search. 

You just need a reasonably big reciever, a methodical search pattern, and a 
lot of a lot of a heck of a lot of patience. Sending out your own signals 
would likely trim down the search time from millions to mere thousands of 
years. Yes, years!. Lightspeed is *so* very slow, when you are 
communicating over interstellar distances. And there are *so* many targets! 
Even if there were a million system-bound civilisations in our galaxy at 
this time, and their distribution was random, the average closest distance 
between neighbours would be some 800 years.