Assume you're an alien civilization flying through space in your fancy high-tech rocket ship, and you stumble upon the solar system. You decide to cruise through, passing each planet searching for signs of "alien life". Your space ship is equipped with a radio telescope, and you point it at all the planets as you fly by. You figure that radio waves are good at carrying information, so any sort of advanced civilization would have figured out how to use them to transmit signals and therefore their planet would glow with radio waves. As you fly by Earth, you point your telescope towards the atmosphere and change any incoming signal to sound waves. What do you hear?
You would probably get a jumbled noisy message of clips from your favorite TV show mixed with blips from songs on XM radio and search results from Google. Right? Wrong! (Even I though you'd get some radio signals from Earth, so don't feel bad!) The reason Aliens won't be able to listen to XM radio and watch TV is because of Earth's Ionosphere. The ionosphere is the very upper layer of Earth's atmosphere , and is basically a mixture of molecules in a gaseous/plasma state. When the sun's rays and the solar wind hit the Earth, it ionizes molecules in the ionosphere. When this happens, radio signal is emitted, and the northern lights are created. This happens so often that the amount of radio signal emitted way overpowers any signal produced by humans that leaks out into space. So if you turned this radio signal into sound waves, you wouldn't hear beautiful music…. you'd hear some pretty awful screeching. Astronomers have actually recorded the noise Earth makes. You can listen to it here: http://www.space.com/10296-earth-alien-sounds.html
