An important fact to note here is that the narrower a signal
(the more refined the frequency), the more efficient it is for our friends
to send. A narrower frequency signal is also easier for us to pick up above
the normal noise level. Unfortunately, very narrow frequency signals mean
that we have to look at millions of very narrow frequencies to find just
the right one. More on this later.
What kind of signals are we likely to receive? What should
we be looking for? There are really two possibilities. Either the other
civilizations are intentionally sending out a signal that is expressly meant
to get our attention, or, like us, they just happen to be doing their own
business and some of their signals are "leaking out." The earth
broadcasts lots of stuff unintentionally. Our radio and television transmitters
are very strong. Our military radars are even stronger. We've been leaking
this stuff for at least 50 years, so our signals are about 50 light-years
out. Not very far, but we are still technological infants. It seems like
we should try to search for the leakage-type signal and assume that we will
also pick up anything that is beamed at us.
What Do We Look With?
As a signal spreads out from its source, it gets weaker
and weaker. The distances between stars are so vast that any signal that
reaches the earth will be very weak (unless it's being beamed at us). To
gather up enough of this signal we need a giant "ear." In our
case, that means using a huge radio telescope.