<a href="http:&#x2F;&#x2F;www.seti.org&#x2F;faq" rel="nofollow">http:&#x2F;&#x2F;www.seti.org&#x2F;faq</a><p>How would we know that the signal is from ET?<p>Virtually all radio SETI experiments have looked for what are called “narrow-band signals.” These are radio emissions that extend over only a small part of the radio spectrum. Imagine tuning your car radio late at night … There’s static everywhere on the dial, but suddenly you hear a squeal – a signal at a particular frequency – and you know you’ve found a station.<p>Narrow-band signals – perhaps only a few Hertz wide or less – are the mark of a purposely built transmitter. Natural cosmic noisemakers, such as pulsars, quasars, and the turbulent, thin interstellar gas of our own Milky Way, do not make radio signals that are this narrow. The static from these objects is spread all across the dial.<p>In terrestrial radio practice, narrow-band signals are often called “carriers.” They pack a lot of energy into a small amount of spectral space, and consequently are the easiest type of signal to find for any given power level. If E.T. intentionally sends us a signal, those signals may well have at least one narrow-band component to get our attention.