“I know perfectly well that at this moment the whole universe is listening to us, and that every word we say echoes to the remotest star.” -Jean Giraudoux
Since the 1930s, humans have been broadcasting radio signals powerful enough to be picked up by a sufficiently advanced alien civilization, even one located many light years away. In 1960, we began monitoring the skies for a similar incoming message, armed with the knowledge that if a civilization were transmitting the same type of signals our radio and TV broadcasts consisted of, we’d be able to unambiguously detect them.
But over the past half century, our technology has changed rapidly, and our radio and TV broadcasts no longer send out as many signals with as much power into space. As the 21st century progresses, the very signals we’ve been looking for are beginning to vanish from our own world. It’s quite probably that if we’re looking for an alien civilization, particularly one more technologically advanced than our own, we’re looking in completely the wrong way.