Used to be a lot of jokes about aliens being able to watch old I Love Lucy episodes and the like due to TV and radio broadcasts beaming out forever into space.
But they may be plumb out of luck when it comes to watching Stargate: Atlantis or newer shows. The Earth is fading to black when it comes to such broadcasts, as more and more “broadcasting” is going from open frequencies to cable/fiber communication that don’t leak signal out beyond our planet.
The same holds true for our detecting communication from other worlds, too, and SETI sorts say that may explain why they haven’t found anything of late. Indeed, the current thinking is that such communication would need to be made by explicit beacons shouting out a Who-like “We are here! We are here! We are here!” as the narrow technology era that pollutes the local radio waves passes.
Which serves as yet another cautionary tale for sf writers who assume certain technological fundamentals will always remain the same …
Nah. The problem is that we haven’t invented subspace or hyperwave radios yet. As soon as we do, we’ll be invited to join the Pan-Galactic Union, or subjugated by the Invincible Overlords, whichever hears us first. I can’t wait!
I suspect the most powerful transmitters are those AM talk radio shows and overseas AM stations, or perhaps Short Wave. I don’t know how well those propagate into space, but I can imagine that if Space Aliens are listening to Rush Limbaugh and his ilk, it may be some time before we hear from them.