Some SETI researchers believe the best way to detect aliens is to search the skies for their laser beams. In the largest survey of its kind, astronomers scanned 5600 stars in search of these optical signals — and they found... absolutely nothing. Nada. Zilch. Here's what that means to SETI and the ongoing hunt for alien intelligence.

The "Sombrero Galaxy". (Image: Hubble)

In a new study accepted for publication at the Astronomical Journal, SETI astronomers Nathaniel Tellis and Geoffrey Marcy from the University of California Berkeley report that they were unable to detect optical signatures of advanced extraterrestrials in over 67,000 individual spectra produced by nearly 5600 stars in the Milky Way Galaxy. Revealingly, around 2000 of these stars are suspected of hosting warm, Earth-like planets, suggesting that advanced civilisations either don't get into the habit of transmitting powerful laser beams across the cosmos, or they simply don't exist. More practically, it means we should look for optical signals elsewhere, and expand our search to include an assortment of other potential alien signatures. Simply put, we aren't done searching for ET.

That said, the null result is undeniably discouraging. Laser signals would be an effective, cheap and easy way for an advanced civilisation to get our attention. Using technology similar to what we have today, aliens could deliberately transmit artificial infrared, visible or ultraviolet emissions at our star. These directed signals could attract our attention by being continuous and abnormally powerful, or by containing tell-tale signs of artificiality, such as unexplained pulsing, or a string of binary data expressing some kind of mathematical phenomena (for example, prime numbers or pi).

Prior to this study, SETI researchers had evaluated around 20,000 stars in search of optical signals at Harvard's Oak Ridge Observatory, spending about 10 minutes on each object. Clearly, if the laser blinking frequency is longer than that, or if ET's laser transmission station is temporarily out of service, we're out of luck. Not surprisingly, nothing interesting has been detected thus far.

In an effort to conduct a more thorough review of the heavens, Tellis and Marcy analysed a trove of data collected by the Keck 10-metre telescope and its high resolution spectrometer, HIRES, between 2006 and 2016 as part of the California Planet Search (CPS). The 5600 stars included in the study, the majority of which are located within a distance of 300 million-light years, produced 67,708 individual stellar spectra, averaging 96 spectral signals per star.

"This study leverages the huge amount of data collected by the Keck telescope over decades, mostly as part of planet-hunting projects," explained Penn State astronomer Jason Wright, who was not involved in the study, in an interview with Gizmodo. "That makes it sensitive to relatively low laser powers from thousands of the most nearby and interesting stars. It's a great example of how SETI can 'piggyback' off of other science, looking for signals that might have been overlooked or thrown out because they were not expected or they look very similar to known sources of noise."

Armed with this data, the researchers then set about the task of searching for spectral signatures that, in the words of the authors, "would be expected from extraterrestrial optical lasers." The power of these lasers ranged from 3 kW to 13 MW, which isn't extreme by any measure. Unlike radio signals, which dissipate over vast distances, laser light manages to hold its integrity as it travels through space. "We may imagine that beings more technologically advanced than humanity would be capable of constructing... laser launchers with power levels at least as high as those detectable here, for any of the 5,600 star systems we surveyed," explained the researchers in their study.

An example of a false positive in a signal. This extraterrestrial laser candidate was found in an observation of TW Hydrae. The blip turned out to be heated gas in the protostellar disk around the star. (Image: Tellis and Marcy, 2017).

To analyse this decade's worth of data, Tellis and Marcy developed an algorithm that was (at least in theory) able to discern a possible alien signal within the natural spectra of a star. If an artificial signal was directed towards Earth, it would be detectable as an unusually high number of protons compared with the background emissions of the star. The algorithm was configured to flag any occurrence of three consecutive pixels that exceeded the researchers' thresholds.

"We searched our spectra for 'brightenings' of the star, relative to the light it is already emitting, that were both tight in wavelength and in space," Tellis told Gizmodo. "Finding a signal that matched the instrumental profile of Keck HIRES would have almost unequivocally meant we were seeing laser light, as the normal stellar spectra contain only thermally broadened emission lines. This is one of the key advantages to using Keck, as it has high enough spectral resolution to distinguish the two."

The thresholds were very liberal, resulting in an initial batch of 5023 candidates. The researchers manually parsed these results (literally eyeballing the data), pruning the list down further and further as they pinpointed the source of each false positive. The most common sources of these false positives included cosmic rays, gamma rays, radioactivity from the observatory, molecules in Earth's atmosphere, and emissions from the parent stars. Eventually, Tellis and Marcy had to concede defeat.

"We found no such laser emission coming from the planetary region around any of the 5,600 stars," conclude the researchers in their study.

This result would seem to strike a blow to the suggestion that advanced civilisations might last for thousands or millions of years, all the while sending hello messages to up-and-coming neighbours. If even a small fraction of the 2000-or-so systems with potentially Earth-like planets surveyed had technological civilisations who took the time to deliberately beam megawatt-lasers towards us, we should have detected them by now.

"These results put an upper bound on the number of civilisations transmitting lasers at us while we were observing," said Tellis. "It is only one type of communication, but we believe that for targeted communication, lasers are highly efficient." That said, he admitted that lasers as a communication medium seem good to us at this time due to our relative youth, and that his team's strategy relies on serendipity. "We have to 'catch' their broadcast," he said. "Nevertheless, we believe it is a valuable result that the galaxy is apparently not teeming with such bright lasers."

So either advanced alien civilisations don't behave in this way (for example, they hide their presence or engage in other activities), or they don't exist. It's also possible that technological civilisations are exceptionally rare in the galaxy (both in time and space), greatly limiting the ability of the researchers to detect a signal. As the authors of the new study admit, "We may begin to wonder if arguments along the lines of the so-called Fermi paradox have some merit." Indeed, the eerie silence of space is getting louder with each new attempt to detect alien intelligence.

Undaunted, the researchers are planning on an expanded search. As part of the $US100 million ($133 million) Breakthrough Listen effort, they will now turn their attention to stars that were overlooked in the study, including brown dwarfs and other odd astronomical phenomena. In addition to optical signals, SETI researchers can look for other potential signs of alien intelligence, such as microwave or neutrino emissions, Dysonian megastructures, industrial waste signatures, transiting space habitats, and so on.

If the aliens are out there, we'll find them. Eventually.

[The Astronomical Journal (preprint available at arXiv)]

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