"Then there’s the ongoing shift from broadcast (which necessarily uses a small number of very powerful transmitters) to unicast media like cellphones; there isn’t the slightest chance you could even tell there was a cellphone network on the ground from space, since the frequencies are reused on a radius of less than 25 km; from a lightyear away picking out a single base station would require an unfeasibly large aperture (which would be no good for a sky search unless you had a ridiculously long time to perform it)."
Slashdot Mirror
"Then there’s the ongoing shift from broadcast (which necessarily uses a small number of very powerful transmitters) to unicast media like cellphones; there isn’t the slightest chance you could even tell there was a cellphone network on the ground from space, since the frequencies are reused on a radius of less than 25 km; from a lightyear away picking out a single base station would require an unfeasibly large aperture (which would be no good for a sky search unless you had a ridiculously long time to perform it)."
Copied verbatim from Electron Pusher, Fermi's Non-Paradox