Our search for technosignatures - clear signs of advanced civilizations beyond Earth - takes many forms. Many are driven by the famous Drake equation, which attempts to estimate how many technological civilizations there are in the Milky Way. However, there’s a big fat question mark at the end of that equation in the form of a variable intended to account for the “longevity” of a civilization. And to be clear, that doesn’t mean how long the civilization itself survives. It simply means how long it actively creates a signature that is detectable by our current technology. A new paper, available in pre-print on arXiv from Oxford astrophysicist Brian C. Lacki, argues that, since the chances of us overlapping in time with any such civilization are miniscule, we’re much more likely to find the ruins of a “dead” civilization - and, surprisingly, the best place to do so might be in our own solar system.

A fundamental part of this argument is driven by Earth’s own history. Up until now, SETI has focused on receiving “passive” signals from beyond the solar system, typically in the form of radio waves. However, even on Earth, our own “window” of sending radio signals into the vastness of space only lasted for around 100 years. We are actively eliminating most wide-broadcast radio signals in an effort to improve our communications infrastructure. So, in other words, even our own civilization isn’t bothering to maintain what minimal intentional broadcasts we were producing 50 years ago.

Instead, the argument goes, it’s better to find “passive” technosignatures, such as relics that require literally no upkeep and can last for billions of years. That would eliminate the need for “constant maintenance” of a radio transmitter or high-powered laser, and would make us much more likely to find the types of civilization that could, at least at one point in time, support that.

Fraser discusses our current search for technosignatures.

So what would such a “passive technosignature” look like in practice? Dr. Lacki breaks them down into three categories - diffusers, occulters, and glinters. From our perspective, occulters would be visible from its unnatural dimming pattern, which would appear similar to a transiting exoplanet, but clearly not the same. Glinters, on the other hand, feature gigantic mirrors that can focus or reflect starlight over thousands of light years, appearing as anomalous “lens flares” near their host star. Diffusers scatter light nearly isotropically, creating a faint signal that might reflect an unusual color or polarization.

Any of these systems is entirely passive, and requires no active role from their creators whatsoever. However, simply building enough of them will indeed require some form of maintenance. A Dyson Swarm is certainly within the capabilities of the types of civilizations being considered in this paper, but maintaining the orbital mechanics of such a swarm does involve active intervention, even though it’d be much less than an active radio transmitter.

Without such support, the components that make up the Dyson swarm would inevitably be drawn together via gravity, eventually colliding and creating what Dr. Lacki calls “technograins”. This destruction could even be accelerated by a “chain reaction” effect similar to Kessler syndrome here in Earth’s orbit, with each additional collision creating yet more debris to create yet more collisions. Do this enough times and even an alien megastructure can be ground down to micron-scale dust.

Fraser talks technosignatures with Dr. Jacob Haqq-Misra

Once small enough, these technograins might gain a trip out of its host solar system by a solar wind that overcomes the star’s gravity holding it back. These motes of dust are then free to roam the galaxy, escaping any long term confinement to their host star. That’s where the other interesting idea from Dr. Lacki’s paper comes in.

Our solar system isn’t stationary in comparison to the galaxy. As it orbits the Milky Way, it routinely sweeps through interstellar material, some of which might be made up of pulverized technosignatures. Even if that material swept into our galaxy billions of years ago, inactive worlds like the Moon could preserve it from that original time all the way down to now. In other words, researchers could screen Moon dust for signatures of extinct megastructures.

Ultimately, what the paper points out is that we don’t need bigger and better space telescopes to continue our search for technosignatures. Instead we might be able to find it by sifting through the regolith on our closest neighbor. And if we do find any, it will bring new meaning to the phrase “dust to dust” - since there will be a completely different form of intelligence handling the dust that came from a completely different civilization.

Learn More:

B. C. Lacki - Dust to Dust: Prospects for Passive Technosignatures as Relics of ETI

UT - Galaxies with High Radio Emissions Could be Home to Many Advanced Civilizations

UT - Breakthrough Listen Releases its one-of-Everything "Exotica" Catalog

UT - The SETI Institute Releases Technosignature Report on 3I/ATLAS