I want to believe — but yet another massive search for alien technosignatures just turned up nothing

Hunting for alien civilizations isn't a matter of just waiting around for them to show up; it's the business of combing through enormous volumes of data to look for peculiar signals. The good news is that astronomers have developed an efficient method for doing exactly this. The bad news is that they haven't found anything … yet.

It seems like a somewhat reasonable assumption that if other civilizations are out there in the universe, eventually they will discover how to emit powerful radio broadcasts. Radio waves are capable of traversing great interstellar distances, so they make a great calling card. This is the foundational assumption for the search for extraterrestrial intelligence (SETI). Strange radio signals might be a sign of an artificial transmission from an alien species.

But our Milky Way galaxy is swimming in radio emissions of all sorts, from exploding stars to the vibrational hum of the galaxy's magnetic field. Plus, humanity has developed a particular fondness for radio transmissions, so any radio search for aliens must deal with enormous quantities of human-caused signals.

Previous SETI searches have scanned large areas of the sky and flagged anything interesting that popped up. Then, researchers have combed through the flagged results by hand, searching for signs of artificial transmission while ruling out potential causes of human-made interference.

Previous SETI searches have also come up totally empty — which isn't a big surprise, since this semimanual technique limits how much data any one research team can process.

Enter COSMIC, the Commensal Open-source Multi-mode Interferometric Cluster. COSMIC is a computer and software system that piggybacks on that of the Karl G. Jansky Very Large Array, the iconic radio array located in the desert of New Mexico.

COSMIC is designed to automate the process of SETI searches as much as possible. By combining fast processing and a series of restrictive filters, the system searches signal after signal, deciding if it's likely to be artificial and, if it is, determining if it matches the signature of a known terrestrial source.

In particular, COSMIC searches for radio signals that are very tightly focused, suggesting that they come from a very small source, like a planetary surface. Next, it looks for Doppler shifting of that radio signal. If the signal comes from a planet, the motion of the planet will either redshift or blueshift the signal, depending on whether the planet is moving away from or toward us when the signal was emitted. If the system finds a signal matching these properties, it is flagged and advanced to the next stage of filtering.

Related: How AI is helping us search the universe for alien technosignatures

Next, the astronomers behind COSMIC know the properties of terrestrial radio emission. This unwanted artificial emission follows particular statistical properties. If the flagged signal of interest matches those properties, the signal is rejected. Any remaining signals are then flagged for further review.

The COSMIC system is a part of the VLA Sky Survey, which completed a scan of roughly 82% of the Northern Hemisphere's sky. All told, the COSMIC system analyzed over 950,000 individual pointings of the telescope. Although the system initially flagged thousands of potentially interesting signals, none survived all of the filtering steps.

In other words, a deep radio search of a good chunk of the Northern Hemisphere found no artificial radio signals.

Although this is initially discouraging, this result still represents an important advance in our search for alien life. We can use this data to narrow down the probabilities of life appearing on any one planet, and we now have a valuable tool for collecting and processing data in future surveys, which might turn up something much more interesting.