Searching for extraterrestrial signals is like trying to hear a whisper at a roaring stadium. The Green Bank Telescope listened to 70,000 stars, but 100 million bursts turned out to be earthly noise — from phones, satellites, microwave ovens. Automation filtered out almost everything, and the rest was checked by 40,000 volunteers. There's no alien whisper.
Silence itself is important. The transmitter we were looking for is tens of times more powerful than all the energy humanity has ever emitted. We'd have noticed it with ease. But there are fewer than six such 'horns' per 100,000 stars within 20,000 light-years. The stadium is empty.
The total energy of all human radio broadcasts ever made is less than the power of a single strong transmitter sought by this telescope.
In the movie 'Contact,' radio telescopes pick up an extraterrestrial signal. The real search, so far, is more like an attempt to hear silence.