A group called SETI (Search For Extraterrestrial Intelligence) attempts to find evidence of extraterrestrial intelligence by analysing electromagnetic radiation hitting the earth from outer space. They hope to catch stray radio broadcasts, communication signals or some-such.

Unfortunately any sufficiently advanced form of communication resembles noise. Consider a signal going down a telephone connection. When phones were just used for voice they were easy to hear and recognize. But when that line is used for computer connections, it sounds very much like noise (try picking up the phone when a 'dialup' computer modem is connected and listen). In order to pump as much information down the line as possible, the entropy is increased, and the signal just resembles noise.

Digital TV signals will also very closely resemble noise, and in a few decades, nearly all electromagnetic broadcasts emanating from the Earth will also just resemble noise. If someone has the decoder, it makes sense, but to an observer just listening without 'inside knowledge', it is indistinguishable from noise.

So listening for recognizable elements in signals emanating from intelligent beings is just going to pick up noise unless we happening to be listening not just to the right place, but also at the right time - perhaps in the few decades between the initial use of the medium, and the inevitable advanced use of it.

SETI faces a seemingly insurmountable challenge.