In radio astronomy, there are lots of natural radio signals to observe. The glow of hydrogen gas, the swirl of electrons along a magnetic field, or the pop-pop-pop of pulsars. These signals usually have a very natural character to them, so astronomers can distinguish them from the artificial chirps and chatters of terrestrial sources. But when you're looking for the signals of alien civilizations, things can get more tricky. They should have an artificial character similar to the radio signals of humans. So how can astronomers distinguish between the distant artificial signal and the local ones?
