The Search for Extraterrestrial Intelligence (SETI) is usually about collecting data. Researchers look at broad ranges of the electromagnetic spectrum for evidence of intelligent life, in hopes that an alien civilization has thought to send an interstellar signal our way. But in a few cases, humans have turned the tables, sending out messages from Earth in hopes of catching aliens' attention. This is Active SETI.
In the slideshow below, we've compiled a few well known attempts at Active SETI. While the efforts have been largely symbolic, the messages themselves and the logic behind them can be fascinating. Many people have heard of the Pioneer Plaque, with its images of a nude man and woman, but what about the time when NASA beamed the Beatles' classic 'Across The Universe' into space, or the broadcast that included data on humanity's latest mathematical achievements?
In the wake of NASA's observation that the planet Kepler-22b may support life as we know it, SETI researchers have begun direct their attention toward 'Earth's Twin.' It may only be a matter of time before Active SETI proponents begin sending messages on behalf of the human race.