The Search for Extraterrestrial Intelligence (SETI) is a collective effort by independent organizations, government agencies, educational institutions, and individuals to determine if intelligent life exists outside of the confines of our tiny, blue planet.
Whenever extraterrestrial intelligence (ETI) is mentioned, almost invariably people start to think of flying saucers and little grey (formerly green) men. It is important to make a distinction between SETI and "paranormal" research. SETI researchers are concerned with the possibility of the existance of ETI, not whether ETI is visiting earth.
If intelligent life exists outside of earth, then it is almost certainly rare. In 1960, Dr. Frank Drake proposed a formula to calculate the number of intelligent civilizations in our galaxy. This formula is now known as the Drake equation. Unfortunately, no exact result of the Drake equation can presently be reached. This is because many of the factors in the equation, such as the average rate of star formation, are not fully known. When Drake calculated the equation, he determined that there could exist 10 intelligent civilizations in the Milky Way. Based on most currently accepted estimates for the variable values, the result is closer to 2.
Two possibilities exist: Either we are alone in the Universe or we are not. Both are equally terrifying. - Arthur C. Clarke
In either case, most people at some time or other have looked at a clear, night sky and wondered, "Are we alone?" SETI hopes to, one day, be able to answer that question.
Over the years, a number of methods have been proposed to accomplish the mission of SETI. Typically, these approaches are categorized into either passive SETI or active SETI. Which method is most "appropriate" is the subject of considerable debate.
Passive SETI relies on the possibility that ETI may contact us in some way and, as a result, we open ourselves to receive such communication. The most common form of passive SETI involves "listening" for radio signals originating from outside of the earth. Radio communication, such as those broadcast by television and radio stations, takes the form of a waves that radiate out from a broadcast point. Your favorite radio station emits waves from a large antenna, and those waves spread out in all directions. Your radio "listens" for these waves and converts them back into something you can audibly hear and understand. But these waves don't stop at your radio. In fact, once radio waves reach the vacuum of space, they just go on and on forever. Within just a few minutes, television broadcast (though in need of significant amplification) could be picked up on Mars! Large radio observatories, such as "Big Ear" or the Arecibo Observatory, are configured so that if radio waves produced by a civilization somewhere else ever collide with earth, then we'll hear them!
Active SETI, on the other hand, does not wait for communications from other civilizations to reach earth. The goal of active SETI is to produce communications that may one day be heard by other civilizations. The goal of active SETI is to alert other civilizations to our presence. Using the same principles as passive SETI, specifically that radio waves can continue indefinately in a vacuum, specific radio waves that expose human intelligence and location are broadcast into space in the hopes that someone "out there" is listening through a passive SETI program. In 1974, the Arecibo Observatory was used to broadcast a high-powered transmission in the direction of a star cluster located approximately 25,000 light years away. As of 2008, this message is approximately 1/10th of 1 percent (0.1%) of the way through its voyage.
However it is accomplished, both passive and active SETI methods continue toward a common goal - to determine the answer to the question that we all want to know: "Are we alone?"