In part five of Astronomy Now‘s fifty years of the Search for Extraterrestrial Intelligence (SETI) series of articles, the magazine examines why there are certain regions of the electromagnetic spectrum that are believed to be likely to be used by those wishing to send a signal across the cosmos. These include the ‘watering hole’ a region between the 18cm emission of HO and the 21cm emission of H where various background noises prevalent at other wavelengths are at a minimum. It is quiet and so any signal comes through loud and clear.
Another outlook is to use something a little clever – like Pi (a universal ratio) times the frequency of the nearest pulsar to a civilisation (or even Pi times the 21cm radio frequency). Both the waterhole approach and the pulsar approach have led to the detection of a signal (as has optical SETI, which looked for bright laser signals in stellar spectra) but none has ever obtained a repeat signal, which could indicate something interesting is going on.
More information on this methodology can be seen in the article itself, but there’s one guy who won’t be picking up the phone if ET does call our home. The cosmologist Professor Stephen Hawking has declared the likelihood of hostility with aliens too high to contemplate making contact with them. He fears a repeat of such episodes as Columbus meeting the natives on arriving at America, and believes aliens travelling through the galaxy are likely to be asset strippers.