Aerial view of the Arecibo Observatory in Puerto Rico

Photo courtesy of the NAIC - Arecibo Observatory, a facility of the NSF

Testing and Revising the Drake Equation

Armed with an estimate of the number of communicative civilizations in our galaxy, SETI scientists set out to find them. They had two basic options: face-to-face communication or long-distance communication. The former scenario required that extraterrestrials visit humans or vice versa. This seemed highly unlikely given the distances between our solar system and other stars in the Milky Way. The latter scenario involved radio broadcasts, either sending or receiving electromagnetic signals through space.

In 1974, astronomers intentionally transmitted a 210-byte message from the Arecibo Observatory in Puerto Rico in the hopes of signaling a civilization in the globular star cluster M13. The message contained fundamental information about humans and our corner of the universe, such as the atomic numbers of key elements and the chemical structure of DNA. But this sort of active communication has been rare. Astronomers mostly rely on passive communication -- listening for transmissions sent by alien civilizations.

A radio telescope is the tool of choice for such listening experiments because it's designed to detect longer-wavelength energy that optical telescopes can't see. In radio astronomy, a giant dish is pointed to a nearby, sunlike star and tuned to the microwave region of the electromagnetic spectrum. The microwave frequency band, between 1,000 megahertz and 3,000 megahertz (MHz), is ideal because it's less contaminated with unwanted noise. It also contains an emission line -- 1,420 MHz -- that astronomers can hear as a persistent hiss across the galaxy. This narrow line corresponds to energy transformations taking place in neutral hydrogen. As a primordial element of the universe, hydrogen should be known to all intergalactic civilizations, making it an ideal marker. Several teams from around the world have been systematically listening to stars across the Milky Way and adjacent galaxies since 1960.

Despite their collective efforts, no SETI search has received a confirmed, extraterrestrial signal. Our telescopes have picked up a few unexplained and intriguing signals, such as the so-called "Wow" signal detected by researchers at Ohio State University in 1977, but no transmission has been repeated in such a way that it provides indisputable evidence of extraterrestrial life. All of which brings us back to the Fermi Paradox: If thousands of civilizations in the Milky Way galaxy, why haven't we detected them?

Since Drake and Sagan made their estimates, astronomers have become more conservative. Paul Horowitz, who boldly guaranteed the existence of extraterrestrial life, has generated more modest results from the Drake Equation, finding that N may be closer to 1,000 civilizations [source: Crawford]. But even that figure may be too large.

In 2002, Skeptic magazine publisher Michael Shermer argued that astronomers weren't being critical enough in their evaluation of L, the length of time a civilization remains detectable. Looking at 60 civilizations that have existed on Earth since the dawn of humanity, Shermer came up with a value for L that ranged from 304.5 years to 420.6 years. If you plug these numbers into the Drake Equation, you find that N equals 2.44 and 3.36, respectively. Tweak the numbers some more, and you can easily get N to fall to one or even lower. Suddenly, the odds of hearing from an extraterrestrial life form are considerably lower.

Even the most enthusiastic SETI supporters are troubled by the lack of results produced by more than 40 years of "listening" to the cosmic airwaves. And yet most of that search has been confined to our home galaxy. Even if there are only three or four civilizations per galaxy, there are billions and billions of galaxies. This tilts the odds again in favor of finding extraterrestrial life, which is why many SETI astronomers take the same approach to their work as lottery players: You can't win if you don't play.