In particular, light pollution appears to be the clearest sign of an advanced civilization. "Observed over interstellar distances, they would reveal to the observer the presence of a technology," say a team of astronomers led by Jean Schneider of the Paris Observatory at Meudon, France. Still, even if we left all the lights on and pointed them directly into space, the equipment used to detect this anomaly -- and filter out light from other naturally occuring sources -- would have to be massively advanced. From New Scientist:
"To reliably detect even this massive amount of artificial light on a planet orbiting a relatively nearby star -- say 15 light years away -- would require an array of telescopes with a combined light-collecting area of 1.5 square kilometres, Schneider's team calculates."
In addition to light pollution, gases from industrial processes might also make certain planets stand out more than others. "The chemicals known as CFCs strongly absorb infrared light at characteristic wavelengths, making them detectable in the atmosphere even when present at concentrations of only parts per trillion," New Scientist reports. "CFCs do not form naturally, so detecting them on a world orbiting another star would be good evidence of alien technology."
Of course, due to the way CFCs were destroying our ozone, they're no longer allowed in the manufacturing process -- so that's one calling card that won't be around for much longer. Still, I have to wonder if pollution might also just cause some wise aliens to skip right over us. I mean, what "advanced" civilization that poisons itself is worth contacting?
Source: New Scientist and http://www.mnn.com/