Marvin <kitfox@pixie.co.za> wrote in message news:<4093f252.0@news1.mweb.co.za>...

For you information, a 1 watt, systemwide-focused, monochromatic 
transmission will outshine the sun at anything more than about half a 
lightyear. At 100 lightyears distance, that 1 watt will outshine the sun's 
radiation at the same frequency by 5 magnitudes! The only real limit 
applicable here is how well they can focus the transmission, and this is 
directly related to how big they are willing to make the transmitter. A 
reasonably advanced civilisation using a solar-system scale transmitter 
(easily done witha few dozen coordinated emitters) can achieve solarsystem-
scale focus up to some 50 000 ly.

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.

Another way to see this is that our radio telescope arrays can resolve
features 10 AU big at 50,000 light years.   So if run in reverse as
transmitters, they could create a beam of the same size (assuming, of
course, that we solved lots of practical problems such as UV plane
coverage and difficulties of phase referencing through the ionosphere
while transmitting).

   Lou Scheffer