writes "Forget radio signals. Two scientists, Abraham Loeb, of Harvard University and Edwin Turner, from Princeton University, have said it may be possible with the next generation of telescopes to pick up the lights from cities on alien planets. On Earth, city lights are so bright they can be seen from space — and their spectral signature differs from that of the gases in the atmosphere and the sun. If one were looking at an alien civilization, one would expect to see the same thing.
The reason they proposed this is that aliens may not generate as much radio energy as their technology improves, given that on Earth we bleed less radio energy into space as we have moved to fiber optics."Link to Original Source