In this article on Astrobiology Magazine, Charles Q. Choi examines researchers' theory that alien civilizations on distant planets could be detected by their cities' lights that would illuminate their nights. That is, assuming aliens meed to see at night like we do.
"Both Ed and I were attending a conference in Abu Dhabi about novel ways to detect life, and we had a tour guide on a trip to the nearby emirate of Dubai who bragged that it was so bright at night that you could see it easily from space -- that's what gave us the idea," Loeb told Astrobiology Magazine.
On Earth, artificial illumination comes in two forms -- thermal, in the form of incandescent light bulbs, and quantum, in the form of fluorescent lights and LEDs. The spectra or combination of colors from this artificial lighting would likely differ from natural sources of light such as volcanoes, and thus might serve as a lamppost that signals the existence of extraterrestrial technology and intelligent life.
This new tactic is similar to traditional SETI (search for extraterrestrial life) strategies that hunt for alien radio transmissions. However, given how radio emissions from Earth have declined dramatically in recent decades due to the use of cables, fiber optics and other advances in telecommunications technology, a switch to looking for extraterrestrial light pollution might not be such a far-fetched idea after all, the researchers suggested.