Giant mirrors in space have been a staple of science fiction for decades. But so far, there's been very little work looking at the actual physics behind the concept, possibly because we're still so far from making them ourselves. Still, they could potentially serve as a passive technosignature if we manage to find one. In order to do that, though, we have to understand what we're looking for. That is the purpose of a new paper, available as a preprint on arXiv, by Shauna Sallmen of the University of Wisconsin, La Crosse and Eric Korpela of UC Berkeley.