Photo courtesy NAIC-Arecibo Observatory, Photographer David Parker
Arecibo Radio Telescope
Are we alone in the universe, or are there intelligent beings out there with whom we could communicate? We may never know if we rely on space travel -- distances between the stars are unimaginably vast, and our most advanced ideas for space rockets, such as light propulsion, nuclear propulsion, solar sails and matter-antimatter engines, are many years away from becoming reality.

How can we detect signs of extraterrestrial (ET) life? One way is to basically eavesdrop on any radio communications coming from beyond Earth. Radio is not only a cheap way of communicating, but also a sign of a technological civilization. Humanity has been unintentionally announcing its presence since the 1930s by way of the radio waves and television broadcasts that travel from Earth into outer space everyday.

The Search for Extraterrestrial Intelligence (SETI) is conducted by dedicated scientists everyday. In the movie "Contact," Jodie Foster's character, Ellie Arroway, searches the heavens with several large radio telescopes. When she receives a radio message from a distant star, there are profound implications for humanity.

SETI is an extremely controversial scientific endeavor. Some scientists believe that it is a complete waste of time and money, while others believe that detection of a signal from ET would forever change our view of the universe. In this edition of HowStuffWorks, we will examine the SETI program. We'll look at how radio telescopes work and how they are used for SETI searches, what the probabilities of detecting alien life are, what might happen if or when such a signal is detected and how you can participate in SETI yourself.