Maybe we don’t need bigger and better space telescopes to continue the search for technosignatures, but rather a good sieve.
Our search for technosignatures — clear signs of advanced civilizations beyond Earth — takes many forms. Many are inspired by the famous Drake equationwhich attempts to estimate how many technological civilizations there are in the Milky Way.
However, there is a big unknown at the end of this equation, in the form of a variable designed to take into account the “longevity” of a civilization.
And to be clear, this does not mean how long civilization itself survives. It just means how long it actively produces a signature detectable by our current technology.
A new article, in pre-publication on arXiv and signed by the astrophysicist Brian C. Lackifrom Oxford, argues that, as the chances of us coinciding in time with such a civilization are minuscule, It is much more likely that we will find the ruins of a “dead” civilization.
Surprisingly, the best place to do this may be our own Solar System.
Look for “passive” signals
A key part of this argument is based on Earth’s own history. Until now, SETI has focused on receiving “passive” signals from outside 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 about 100 years. We are actively eliminating most widely broadcast radio signals in an attempt to improve our communications infrastructures.
In other words, not even our own civilization can be bothered to maintain the minimal intentional transmissions it produced 50 years ago.
Instead, the argument goes, it is preferable to look for “passive” technosignatures, such as relics that require literally no maintenance and can last billions of years.
This would eliminate the need for “constant maintenance” of a radio transmitter or high-power laser, and would make it much more likely that we would find traces of the types of civilization that, at least at one point in time, might have supported something like this.
But what would a “passive technosignature” look like in practice? Lacki divides them into three categories: diffusers, concealers and shimmering reflectors.
From our point of view, the occulters would be visible through their artificial dimming pattern, which would resemble the transit of an exoplanet, but without being exactly the same.
Twinkling reflectors would have gigantic mirrors capable of focusing or reflecting starlight across thousands of light years, appearing as anomalous “lens flares” near the host star.
Diffusers disperse light almost isotropically, creating a faint signal that could reveal an unusual color or polarization.
Any of these systems are entirely passive and require no active role from their creators. However, simply building enough of them would indeed require some form of maintenance.
A Dyson sphere swarm is certainly within the reach of the types of civilization considered in this article, but maintaining the orbital mechanics of such a swarm involves active intervention, even if much less than that required by an active radio transmitter.
Without this support, the components that form the Dyson swarm would inevitably end up being attracted to each other by gravity, colliding and creating what Lacki calls “technograins”.
This destruction could even be accelerated by a “chain reaction” effect similar to Kessler syndrome in Earth’s orbit, where each new collision creates more debris, which in turn causes even more collisions. Repeated enough times, even an alien megastructure can be reduced to micrometer-scale dust.
When they become small enough, these technograins can be expelled from their solar system of origin by a stellar wind capable of overcoming the star’s gravity. These dust grains are then free to roam the galaxy, escaping any prolonged confinement to their host star.
This is where another interesting idea from Lacki’s article emerges.
Our Solar System is not stationary in relation to the galaxy. As it orbits the Milky Way, it regularly passes through interstellar material, some of which could be composed of pulverized technosignatures. Even though this material entered our galaxy billions of years ago, dormant worlds like the Moon could preserve it from then until the present day.
In other words, researchers could analyze lunar dust for signatures of extinct megastructures.
Ultimately, what the paper underlines is that we don’t necessarily need bigger and better space telescopes to pursue the search for technosignatures.
Instead, perhaps we can find them by sifting through the regolith of our nearest neighbor.
And if we ever find these traces, the expression “dust to dust” will take on a new meaning — because it will be a completely different form of intelligence handling the dust left by a completely different civilization.
