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When it comes to detecting laser pulses aliens might shoot at Earth to attract our attention, scientists now find they can detect signals as faint as a single photon of light every few tiny fractions of a second.

Astronomers have gazed at the skies for decades searching for signs of extraterrestrial intelligence. Lasers can in principle help transmit messages over extraordinary distances, but while scientists have monitored a large number of stars looking for alien laser signals — for instance, facilities at Harvard and Princeton scanned more than 10,000 sun-like stars for several years — no evidence for any have been found yet.

Prior attempts to look for extraterrestrial laser signals concentrated on isolated bursts of light, ones so extraordinarily intense they are likely artificial. In contrast, laser scientist Walter Leeb at the Vienna University of Technology in Austria and his colleagues are focusing on repetitive, faint laser signals received over a sufficiently long amount of time.

"We assumed that aliens would use the simplest possible way of attracting our attention, one already implemented in seafaring since ancient times using lighthouses — that is, periodic light pulses," Leeb said.

The kinds of strings of pulses the researchers are looking for, each roughly nanoseconds or billionths of a second long, are not likely found in nature. "Such signals can, however, be generated by lasers," Leeb said, thus hinting at an extraterrestrial intelligent origin.

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