Given what we know, what would be the most efficient way for a civilization to spread its technology across the galaxy? A Georgian physicist, Zaza Osmanov, believes he has the answer. A self-replicating swarm of micro-probes. And we could potentially detect them.
Osmanov’s discussion starts with consideration of Von-Neumann probes, a self-replicating spacecraft. These have been a staple concept in both serious scientific discussion and science fiction. They are after all one of the most effective ways to perform mining operations in outer space. Osmanov played with the architecture of these probes and considered a microscopic version of them.
The paper, which is available on the pre-print server arXiv, calculates that if a civilization's goal is to expand into the cosmos, macro-sized Von-Neumann probes are not very efficient. You need to land on a rocky world, mine it, and launch the probes back into space. Osmanov instead considers tiny probes that can replicate and be fueled simply by the material collected in interstellar clouds.
These replicators would, according to Osmanov, be extremely efficient. They would also be tiny, just nanometers in length. Although their architecture is left as an exercise for a future reader, Osmanov’s calculations conclude that if you start with 100 of these probes and they do nothing but replicate and travel forward through one parsec (3.2 light-years) of interstellar clouds, you will end up with 1033 micro-probes or 1,000 quintillions. That's a few hundred thousand times the number of atoms in the average human body.
This swarm of micro-probes would change the luminosity of the surrounding cloud in a matter of years and the author argues that if we were to see something peculiar happening in a cloud in interstellar space, a civilization hunting for material could be behind it.
“We have analysed efficiency of micro-scale Von-Neumann probes versus macro robots and we found that the former might efficiently self-reproduce in the interstellar media whereas the large scale automata can replicate only on rocky planets, requiring additional maneuvering,” Osmanov concluded in the paper. “All the aforementioned results indicate that if one detects a strange object with extremely high values of luminosity increment, that might be a good sign to place the object in the list of extraterrestrial Von-Neumann probe candidates.”
Achieving this wouldn’t be a walk in the park. Osmanov expects that the endeavor of “mining” an interstellar cloud would be the pursuit of a Type-II civilization planning to stay inconspicuous. According to the Kardashev scale, a Type-II civilization uses and controls energy at the scale of its own star system.