Astrobiology has long been split into two camps: a search for "biosignatures" and a search for "intelligence." These look for very different things, but they also leave a huge gap in between. It took 3.5 billion years for us to go from the first microbe to a civilization that sent radio waves into the cosmos. Detecting life in between those stages is a relatively untouched aspect of astrobiology—which is also the focal point of a new paper, "Signs and Signatures of Intelligence," available on the arXiv preprint server, by astrobiologist Julia DeMarines.
Before we get into that missing middle ground, we should review the two typical astrobiological categories. Biosignature searches focus on chemical traces like oxygen and methane that suggest biological activity. By contrast, "technosignatures" represent the observable products of advanced technology—like radio waves or massive planetary-scale engineering projects.
Civilizations don't just pop up from microbes and start emitting radio waves, though. It's an evolutionary process that takes billions of years. If an alien civilization had turned a telescope toward Earth 10,000 years ago (or alternatively, is viewing Earth from 10,000 light-years away), it wouldn't have seen any radio waves. But it also wouldn't have been looking at a world covered only in simple microbes. So how do we quantify this "middle ground" and incorporate it into our larger study of astrobiology?
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