If E.T. does try to phone us how will we know it? Find out on this Moment of Science.

I was listening to the radio last night, and at first everything was all static-y, and then I heard this "whoo oooo ooo noise". For a moment, I thought it might be aliens, but it was just some guy singing opera. Still, it got me wondering--how will scientists listening for a transmission from outer space know that they've actually picked up a signal?

Well, according to the SETI institute, which helps coordinate the search for extra- terrestrial life, most radio experiments that look for transmissions from outer space focus on what are known as "narrow-band" transmissions. The big assumption here is that any extraterrestrials sending out a transmission are decent engineers, and so they'd send out a focused signal that doesn't take up a lot of bandwidth.

Think about how radio signals operate. You know that electromagnetic waves have certain frequencies, and that when you listen to your public radio station, you're tuning a specific frequency so you can pick up a specific signal.

Well, in contrast, electromagnetic waves that are generated by natural phenomena like pulsars and quasars occur over a broad range in the spectrum, and not just at a narrow frequency. So they show up, so to speak, like static all over the radio dial. Therefore, scientists assume that if a transmission only uses up a small part of the spectrum, it must be artificially generated.