Here's a question I hear quite a bit: why haven't we made contact with intelligent extraterrestrials - ETs?
Physicist Enrico Fermi asked this important question back in the 1940s as a way to demonstrate that we might be alone in the universe - it's now known as Fermi's Paradox. Fermi's Paradox simply states that "the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for, or contact with, such civilizations is a puzzle."
If you discount numerous UFO and alien-abduction stories reported since the late 1940s - some fascinating, but all without a shred of proof - there isn't much reason to suggest that intelligent species exist beyond the Earth. On the other hand, the universe is vast; intelligent civilizations may be widely separated among the less common, Sunlike stars.
The idea of communicating with ETs began in earnest during the 1960s. Researcher Frank Drake, considered the father of SETI astronomy (SETI, short for Search for Extraterrestrial Intelligence), hosted the first SETI astronomy conference in the Appalachian Mountains of West Virginia in 1961.
Around the time of the SETI conference, Drake used the big 26-meter diameter radio telescope of the Green Bank, W.Va., observatory to study two Sunlike star systems: Epsilon Eridani and Tau Ceti.
Following the 1959 suggestion of Cornell University physicists Giuseppe Cocconi and Philip Morrison, Drake had proposed listening to these two nearby stars at the 1,420 megahertz frequency (1 megahertz is one million cycles per second), the so-called magic frequency of the 21-cm line of neutral hydrogen.
This location on the radio dial is now considered the universe's "waterhole" - the radio frequency at which intelligent species might consider a common place to talk and listen, much like the waterholes of Africa where animals come to drink (and humans came to hunt!).