Our curiosity about intelligent life beyond Earth is not a recent inquisition. Humans as far back as Plutarch in ancient Greece (circa 45-125 A.D.) have pondered the existence of entire civilizations existing on worlds far from our sight. However, we have yet to obtain any conclusive evidence supporting the idea that we are not alone in the universe. The search for extraterrestrial intelligence (SETI) has become a formal endeavor only in the past few decades. Many SETI projects, some privately funded, lie just over the horizon and may very well bring us the first scientific evidence in support of alien life.
The SETI Institute, a non-profit and privately funded research lab, stands at the forefront of SETI research. Since its inception in 1984, SETI scientists have been limited in their search by time, money, and technology. One of their largest obstacles has been the lack of a single telescope dedicated completely to the search. SETI scientists have had to conduct their research in piecemeal steps, taking advantage of what telescope time they could, and piggybacking on other scientists' astronomical investigations when possible.
A new age of SETI projects is promising to change all of that. The inception of a new project, the Allen Telescope Array (ATA), is an extremely promising opportunity for SETI researchers. The ATA will be comprised of approximately 350 6.1-meter radio telescopes. The total collecting area of all these telescopes will exceed that of a telescope with a diameter of 100 meters. Located at the current site of the Hat Creek Observatory, the array will provide the chance for researchers to make a dedicated search of the sky for 24 hours a day, 7 days a week. The premise of radio SETI is that the sky is quiet in radio waves, since stars don't emit them, so pulsed radio signals are a telltale of a technological civilization trying to communicate.
Unlike the ATA, which will be gathering radio signals from the sky, Optical SETI will be looking in the optical portion of the electromagnetic spectrum. Specifically, telescopes dedicated to Optical SETI research efforts will make use of three types of light detectors. These light detectors, called photo multipliers, will search for bright and brief (less than a billionth of a second) pulses of laser light coming from another civilization. The premise is that we can pulse a powerful laser such that it has as much energy as the Sun seen from afar, but only for a billionth of a second. Unfortunately this means that an extraterrestrial civilization has to be sending a direct signal in the exact direction of our Solar System. The advantage of Optical SETI, however, is that it is highly unlikely that all three photo multipliers will be hit with light within a billionth of a second; therefore the likelihood of a false alarm is low (about one per year).
So far we have discussed projects that will be directly looking for signs of intelligent life. However, there are several missions in the works that will be searching for terrestrial planets similar to Earth. This is useful to the SETI efforts because once extrasolar terrestrial planets have been identified, scientists will have specific targets to listen and look for messages from intelligent life. Three of such missions are Kepler, Terrestrial Planet Finder (TPF), and Darwin. Identified Earth-like planets from these missions will greatly enhance the likelihood of detecting signs of life in the universe, intelligent or otherwise.