Our galaxy alone contains a hundred billion stars, many of which may have planets. Furthermore, radio signals can be transmitted on any one of millions of frequencies. You could easily choose the right star, but tune into the wrong station and miss the message. But the situation was not quite so hopeless. In 1959 Giuseppe Cocconi and Philip Morrison of Cornell University published a trailblazing paper in the British journal Nature, pointing out that not all radio frequencies are equal.
Some special frequencies occur naturally in the universe.
For example, copious radio waves emanate at 1,420 MHz from the many clouds of hydrogen scattered across our galaxy. On the reasoning that alien beings would be well aware of the search problems facing a potential recipient, they might be expected to send any message at a frequency familiar to radio astronomers. Taking his cue from this ingenious proposal, Frank Drake, an enthusiastic young astronomer at the US National Radio Astronomy Observatory, set about using the Green Bank radio telescope in West Virginia to listen in at 1,420 MHz to a couple of promising nearby stars. Dubbed Project Ozma, after the fabled beings in The Wizard of Oz, Drake's fledgling observation program found nothing remarkable, but it did serve to establish SETI as a serious scientific enterprise. Over the intervening 40 years SETI researchers have vastly improved their technology.