[6SJ7]

Listening for optical or radio signals may be as futile as watching for smoke-signals. Advanced alien races, and the probabilities of a large universe suggest they are out there, may use a form of communication which is even now blanketing the Earth. We just don't yet know how to receive it anymore than someone from the Middle Ages would know how to construct an AM radio receiver.

[Laz711]

True, but I think the theory is, that at some point, an alien intelligence decided to broadcast radio (or other EM) signals out into space, either on purpose, or not. They may have used it for years, or even centuries, before something else (might have) come along. If some of the calculations as to the amount of life in the Universe are close, then we would recieve signals, even if the race that created them has moved on, or is long gone.

Suffice it to say, that no one knows. It is impossible to know if this is a good idea or a bad idea, but who can fault anyone for trying? If it doesn't work, then oh well, we are still alone. But, if it does work, then that opens up whole new doors.