NIGHT in, night out, the rhythmic radio signals reach Earth. The slowest of them sound like a nail being hammered into wood, or a shoe being slapped against a post to rid it of mud. Others are more like a stuttering motor stopped at a traffic signal. Some make almost continuous tones, ripe to be combined into cosmic mood music.
Always the same signature tunes, always from the same points in the sky. Small wonder that when astronomers first heard them back in the 1960s, some thought they were messages from alien civilisations.
The signals aren't from ET, however; they are from pulsars. These extreme cosmic objects have been keeping us on our toes for over 40 years, and are poised for their greatest coup yet. Meticulous measurements of pulsars' timekeeping might just solve one of the biggest mysteries of modern physics: the whereabouts ...
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