The search for extraterrestrial intelligence (SETI) has been going for more than 50 years, with ever more sophisticated detection systems and creative ideas about how E.T. might come calling. Astronomers haven’t heard anything yet, but perhaps it’s only a matter of time. Check out what they’ve been looking for, how they would know if they found it and what the aftermath might be.
How to Listen
The universe emits many signals of its own. Black holes send out bursts of radio waves, X-rays and gamma rays. The dusty disks of forming planetary systems shine in infrared waves. Scientists must separate those so-called dumb signals from the smart signals that might come from extraterrestrials. Because of that necessary sifting, they assume that aliens would try to make their messages look different from the natural pings of the universe. In general, astronomers look for two hallmarks of technology.
Frequency compression: Narrowband signals come in on a small range of frequencies, like an individual radio station. Broadband signals spread across a wider range, like a broadcast that contains the whole FM band at once. Natural objects can only make signals so skinny, so if scientists see one that covers a tiny range of frequencies — like a laser or a satellite ping — they know it had to come from technology.
