SETI May Have Figured Out Why We Haven't Detected Any Alien Signals Yet, Explaining At Least Part Of The Infamous Fermi Paradox

First up for the uninitiated, what is the Fermi paradox? First mooted by renowned Italian-American physicist Enrico Fermi, it basically goes like this: Given the vastness of the cosmos, and the trillions of stars in the universe around which life could potentially form, why have we seen no evidence of the existence of alien civilizations, and why (as far as we know) has nobody attempted to get in touch? In short, as Fermi put it, where is everybody?

Since Fermi first posed the question in 1950, there have been many attempts to explain it, ranging from the benign to the absolutely horrifying. For instance, some subscribe to the "Dark Forest" version of the universe, where the great silence can be explained by everybody being too frightened of everybody else to make a peep, lest it gets their own civilization destroyed. 

Far less sinister, it could be that many alien civilizations are trapped and alone on their home planets, unable to explore the wider cosmos, let alone leave their own atmosphere, or develop advanced technology. Another idea is that civilizations are simply too far spread out in space and time to be able to communicate with each other at all.

The search for alien civilizations can be done in a few ways. For example, we could look for alien megastructures created by advanced civilizations in order to harness the energy of their stars (though we should note that Freeman Dyson, of Dyson sphere fame, only suggested the idea as a joke). Or we could look for leaked technosignatures, just as aliens may one day be able to detect our phone signals, or the signals from our Deep Space Network may make it to other star systems. Lastly, we might search for directed signals, sent deliberately to communicate with Earth.

When looking for signals, scientists have focused on those in the narrowband. The sky, quiet though it might look, is a horrible mess of signals, but apart from the signals we ourselves have sent and a few intriguing outliers, natural signals in the cosmos are overwhelmingly broadband, spanning a wide range of frequencies and wavelengths. SETI suspects that an alien civilization would choose to communicate with narrowband signals for several reasons.

"We can’t be sure that extraterrestrial civilisations will choose to transmit narrow-band signals, or even that they will use radio or laser communications at all, but if they are deliberately trying to attract attention, a narrow-band radio or laser signal is a great way to do so," SETI's Breakthrough Listen project explains. "Both are capable of traversing interstellar and even intergalactic distances, and tend to stand out against the background of natural signals primarily because they span just a narrow range of frequencies."

However, a new paper has thrown a bit of a spanner in the works for this idea and suggests that even directed communication may not be too easy to spot. 

The team attempted to investigate how the exoplanetary interplanetary medium (Exo-IPM) of an alien civilization – basically the space in the region of its star – may affect the signals it sends out into the cosmos. Stars themselves aren't exactly quiet, and the researchers wanted to see whether phenomena like stellar winds and coronal mass ejections would broaden out the narrowband signals vital to incredibly long-distance communication.

“SETI searches are often optimized for extremely narrow signals,” Vishal Gajjar, Astronomer at the SETI Institute and lead author of the paper, explained in a statement. 

"If a signal gets broadened by its own star’s environment, it can slip below our detection thresholds, even if it’s there, potentially helping explain some of the radio silence we’ve seen in technosignature searches."

To test this idea, the researchers looked at our own historical narrowband signals, sent by spacecraft through our Solar System and back to Earth. The team quantified the spectral broadening that happened with these signals, noting that the problem was made worse during the solar maximum, when the Sun is much more active.

The team then tried to figure out how other stars – focusing on M-dwarves (dMs), the most abundant star type in the galaxy – would affect any narrowband signals. While the habitability of planets around these stars is debated, the team focused on M-dwarves due to their long lifetimes, giving plenty of time for technologically advanced civilizations to develop. On this front, there is bad news, with the team finding that M-dwarves were the star type most likely to broaden out narrowband signals.

"We show that the broadening can exceed 10–100 Hz for most systems. These levels of spectral broadening are large enough to shift otherwise detectable technosignatures below the sensitivity thresholds of current search pipelines optimized for sub-Hz channels," the team writes in their paper.

"These results suggest that turbulence-induced spectral broadening—particularly in the dynamic environments of dMs—may offer a compelling explanation for the apparent absence of detected narrowband radio technosignatures," they added.

While this certainly makes the hunt for alien signals more difficult, that doesn't mean it is impossible. Really, we may be figuring out a better type of signal to look for.

“By quantifying how stellar activity can reshape narrowband signals, we can design searches that are better matched to what actually arrives at Earth,” Grayce C. Brown, co-author of the study and research assistant at the SETI Institute, added, "not just what might be transmitted".

The study is published in The Astrophysical Journal.