Rather than looking for aliens who use interstellar radio signals to say "hi," an alternative search strategy is simply to spy on any mega-engineering projects that an advanced civilization might be undertaking. Veteran SETI astronomer Jill Tarter calls this strategy "SETT" - the Search for Extraterrestrial Technology.
A new science paper by Duncan Forgan at the University of Edinburgh in Scotland and Martin Elvis at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., suggests we look for evidence of a very ambitious macro-engineering project: the wholesale mining of an asteroid belt. The asteroid material may be mined to build space colonies, solar power satellites or maybe even an entire "ringworld," as imagined by sci-fi writer Larry Niven.
What's more, precious metals are in high demand for technologies such as computers, high-speed networks and mobile phones. So-called "green technologies" of the future, such as hydrogen fuel cells, will also place a demand on rare resources.
The unconsolidated debris from the birth of planets, asteroids provide a smorgasbord of elements and minerals for harvesting. Meteorite samples suggest that large quantities of gold, platinum, iron, nickel, magnesium and silicon, among other elements, are abundant on asteroids.
Space Freight Cars
We have already found asteroid belts around other stars. In fact, the star Epsilon Eridani has three nested belts. NASA's Spitzer Space Telescope has been prolific at picking out the thermal signature of extraterrestrial asteroid belts.
Any growing space faring civilization would recognize asteroids as an abundant source of raw materials. They are the coal cars in a freight train of planets.