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Extraterrestrial civilizations: When all else fails, try Bayesianism

Schematic view of the Milky Way showing six isotropic extraterrestrial emission processes/Claudio Grimaldi, EPFL

From ScienceDaily:

Could there be another planet out there with a society at the same stage of technological advancement as ours? To help find out, EPFL scientist Claudio Grimaldi, working in association with the University of California, Berkeley, has developed a statistical model that gives researchers a new tool in the search for the kind of signals that an extraterrestrial society might emit. His method — described in an article appearing today in Proceedings of the National Academy of Sciences — could also make the search cheaper and more efficient.

The advantage of Grimaldi’s statistical model is that it lets scientists interpret both the success and failure to detect signals at varying distances from Earth. His model employs Bayes’ theorem to calculate the remaining probability of detecting a signal within a given radius around our planet.

For example, even if no signal is detected within a radius of 1,000 light years, there is still an over 10% chance that Earth is within range of hundreds of similar signals from elsewhere in the galaxy, but that our radio telescopes are currently not powerful enough to detect them. However, that probability rises to nearly 100% if even just one signal is detected within the 1,000-light-year radius. In that case, we could be almost certain that our galaxy is full of alien life.

After factoring in other parameters like the size of the galaxy and how closely packed its stars are, Grimaldi estimates that the probability of detecting a signal becomes very slight only at a radius of 40,000 light years. In other words, if no signals are detected at this distance from Earth, we could reasonably conclude that no other civilization at the same level of technological development as ours is detectable in the galaxy. But so far, scientists have been able to search for signals within a radius of “just” 40 light years.

So there’s still a ways to go. Especially since these search methods can’t detect alien civilizations that may be in primordial stages or that are highly advanced but haven’t followed the same technological trajectory as ours. Paper. (paywall) – Claudio Grimaldi, Geoffrey W. Marcy. Bayesian approach to SETI. Proceedings of the National Academy of Sciences, 2018; 201808578 DOI: 10.1073/pnas.1808578115 More.

The good news is, no one can ever prove They’re Not Out There. The bad news is, the search for Them could be obscuring facts about ourselves. Hey, as long as it’s still fun…

See also: At the Guardian: Why are UFO sightings on the decline?

Atlantic writer loses his faith in aliens

Fixing the unfixable Drake equation

Obituary column: By the time we hear from the space aliens, they will be dead

ET still hasn’t phoned Frank Drake


Americans don’t fear the discovery of alien life. So why do some commentators insist they do?


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