August 25, 2013 - 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. [13 Ways to Hunt Intelligent Aliens]
"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.
The advantage of a repetitious signal is that it can readily be distinguished from random noise, even if a large portion of transmitted pulses are lost before they are received. Leeb did note that astronomical objects known as pulsars also give off repetitive light pulses.
"However, their signals have a rather low repetition frequency, and do not consist of nanosecond pulses an extraterrestrial intelligence will possibly use," he said.
Since current technology on Earth
is capable of powerful visible and near-infrared lasers, the scientists assumed extraterrestrials might be as well. They reasoned aliens would choose wavelengths that would stand out from the light from their home stars as well.
Although Leeb and his colleagues assumed that aliens would use high-energy lasers to fire these pulses, they also reasoned the vast distances these signals traveled means these pulses might be very faint by the time they reached Earth. As such, their strategy employs devices known as single-photon detectors, which as their name implies can detect signals even as dim as a single photon, the particle making up light.